the % stated on the cards and how often the effect happens are different
They should be different because of a small sample size. However, if you had 1,000,000 creatures attack with Thorn Shield out, I'm sure that very close to 750,000 of them would be infected.
maybe im just biast because the outcome isnt what i want.
This is the most likely case; either that, or the game's "randomness code" is broken. And it's not.