I have found two different sites guiding how to achieve 100 push ups: HundredPushups.com and Alternative100Pushups.com. However they have different schedule. For example, if you get into the 11-20 pushups round in your first test, numbers of pushups in each set W1D1 in HundredPushups.com are 10 12 7 7 and max (at least 9), meanwhile in Alternative100Pushups.com are 8 9 7 7 and max (at least 8). Do you know why there is a derivation? Which plan should I stick to?
You'll find that there are many paths to the same goal. Both of these choose similar, but slightly different paths toward the goal of 100 push ups. The Alternative plan opts to start with lower volume at first to help you get acclimated to it.
I would use Alterantive100Pushups.com if I'm not used to working my arms and chest, or a rank beginner. I might use HundredPushups.com if I'm acclimated to doing volume work already.
If you aren't sure, just ask yourself if the progression gives you any concerns about being able to finish.