interestingly (and i don't believe this should happen!), each time the trophy page is pulled up, the % chance(s) change.
here are a few examples - trophy anomalies
(5 pics here, look at the % to hit and be hit)
all that was done was open the page, close the page and re-open. this created a spread from 80%-87% to hit and 40%-48% to be hit.
should not happen i'm thinking.