It's not just about having high level plants, you get the zombies hit points added to your score less whatever scoring zone it's in.
My highest this week is almost 1.9m using double L10 witch hazel and enchantmint.
You're using low level plants that take a lot of time to kill something like a gargantuar (if at all).
You're playing against an algorithm at the start of the week then recordings of players of supposedly similar levels to yourself when they have enough.
Witch hazel is an instant kill plant, every 12 seconds or so it will turn a zombie into a fume shroom, or when boosted by enchantmint into a toadstool (an instant kill plant that generates sun) or a hypnoshroom (another instant kill plant).
I have never seen so many plants banned as in this week's tournament.
After last week's horribleness you'd think the devs would give us something better.
If you have low level plants then check out the level 1 mini forum, you might learn something from eieio the barbarian.