This is how it works:
every mod gets 4 secondary stats upgraded in total and each at levels 3,6,9 & 12 but note - it can be the same stat that gets the upgrade and there must be 4 different secondary stats by the time it reaches level 12.
Different coloured mods start at level one with a different amount of secondaries already there:
grey = 0
green = 1
Blue = 2
Purple = 3
Gold = 4
So for example a green level 1 mod will already have 1 secondary stat. you know it will upgrade at 3, 6, 9, & 12. you know it will have 4 secondary stats by the time it gets to 12. Therefore you definitively know that the first stat to get an upgrade will be the one already displayed. Then it must get the remaining 3 stats at 6, 9 & 12.
Similarly if you have a blue mod at level 1 it will have 2 secondary stats. This time you know it can upgrade those stats twice before it displays new stats i.e. at level 3 & 6. Then it will gain it's 3rd and 4th stat respectively at levels 9 & 12. Which of the two stats will get upgraded is random. it could be 1 each or it could be 2 and 0.
The reason this is important is that you can calculate what mod is worth upgrading and taking the risk on. if you upgraded every mod at random, you'd never have any credits.
Usually people look for speed. If you get a gold mod with +5 speed as one of the secondary stats at level 1 you could be in business. It could end up not upgrading the speed secondary stat in any of the 4 increases and will relatively be a dud. or you could get uber lucky and get all 4 upgrades on speed, taking it to 20+ speed secondary. I've only ever seen 1 of those in my arena shard.
I hope this helps. A good understanding of how mods work is probably the most crucial component to being successful in this game.