I think it has to do with the mini-games at the end.
For Moon Base the final mini-game is evenly balanced, both teams have an equally difficult task. As a result, the battle can go either way. (Much like the 5 points leading up to the end of a TT.)
Great White North is also near balanced. This is because the mirrors can be in on or off position and both teams fight for their preferred position. Mirrors turn on when a zombie activates a mirror and stay on until a plant does the same. The zombies do have an advantage though, in this case the length of time the switch needs to be on. What I mean is, it takes less than half the time given for the final point to fully melt the yeti. Meaning for the plants to get a victory they need to not only protect the mirrors but to do so for a longer amount of time than the zombies. Still, the difference is rather small so in my experience it is about a 40 - 60 split for plants and zombies.
Zomburbia is where things start to disbalance. This map also favors attackers but, this time the plants. The problem here is the sheer number of points to protect. Even if one skilled zombie took each point a small team of plants could pick off the points one by one. The big battle happens near the end when there is only one point left to break. If the zombies can get to it fast enough they can blockade it. (This is the only instance I've seen zombies win here.) Even then, this confines the zombie team to a small area, and most rooms have several entrances. This means a single well placed bean bomb could win the game for plants. Additionally, the activation timers are so small a skilled citron can activate the final point while taking fire. In the end, zombies win maybe 20% of the time. (Probably less) For awhile, I hadn't even seen the zombie victory scene. To this day I've seen it maybe 3-4 times out of all the matches that made it to the final point.
The last two are very uphill battles for the attacking team.
Zombopolis as pointed out is a challenge for plants. Unlike Zomburbia there is only one real point for the zombies to protect. To make their job easier there are very few entrances, and the battery has a significant amount of HP. Even left undefended it takes awhile for the plants to shut it down. To make things even easier for the zombies, each break point on the robot has limited HP. This means the front three are expendable as long as the one in the back is protected. (Conveniently, located right next to the battery.) Lastly, the limited window to attack the robot vitals only compounds the difficulty for plants. To have a chance of winning the plant team needs to be mostly alive during the window and all need to attack with high force. If plants are respawning during this window the match is likely to be lost. Still, the split in my experience isn't much worse than Zomburbia. I would say plants win maybe 15% of the time. Not good odds but still much better than the last map.
Time Park, as in time to give up on the last point. I have seen this map won by zombies exactly once in the entire time I've played. (And honestly, the plants were really dropping the ball that game.) On this map zombies have a really uphill battle. (Literally) First, the bootering ram moves at the same speed with one or a dozen zombies. This means there is no advantage to being in a group, even worse it groups zombies together as easy targets. If getting to the gate weren't enough, (like the golf ball in GW1) zombies also have to activate two levers. This task is made worse by the fact that zombies must take a cannon to reach the levers meaning groups of zombies are hard to gather. One or two plants guarding each point can make quick work of any zombie that makes the gap. (Sometimes just potato bombs and spikeweeds are enough to do the job.) So the bridge is down and the boot is near the gate. Now you have to activate the slow boot timer three times in what little time you have left. All while dealing with the rain of fire the plants are focusing into the room with only three ground floor entrances. Plus, to activate the boot the zombies have to be next to it. Meaning the plants need only to concentrate fire on one spot to win. I can't give an accurate percentage here since I've only seen it happen once out of who knows how many matches.
I know that is a lot of text but, it had a point. Yes, the final points for some maps heavily favor defenders. But, there is balance in unbalance. Plants win Time Park almost always just like zombies win Zombopolis nearly always. The other thing to realize is maps with heavily favoring final points are often easier to reach than those with balanced final points. Meaning, while a final point may be balanced it is far less likely the team will reach it. The big exception here seems to be Moon Base where games can go either way the whole map through. Of course, everything here is based on overall team skill. I'm sure a skilled team can win on any map as the attackers. (Except maybe Time Park, that takes a poor team of plants too.)
You are right that many players don't know what to do on the final points. Sadly, nothing said here or on any other GW2 forum will reach the majority of younger players. (Who are also the majority of the player base.) It would take Pop Cap putting some kind of in game text to fix that. Unlike GW1 the final points are a lot less intuitive. The sad irony is microphone chat would make teaching others much easier, but most people mute their microphones due to the sometimes annoying children. (The very people who need to learn how to take the final point.) At least some players such as yourself know what to do, guess it is just getting lucky and hoping your teammates also know. Because knowing is half the battle.
