Accuracy reduction in Generation II
(Relevant code is here.) Move accuracy in this Generation should be out of [0,255] (not [0,100]); for simplicity reasons, we seem to never mention that anywhere. They start off with that value (e.g. 255 for Pound), then incorporate stat stages (let's say it's -1 accuracy, so it'll end up 255*75/100=191). Then, BrightPowder just subtracts 20 (191-20=171). A random number [0,255] is compared to that number, and if it's greater or equal, it'll be a miss (85/256~=33.2% in the example).
The wording from when this was created was just plainly wrong—the "penalty" didn't decrease "from 20% to 10%"; it was additive, then became multiplicative. However, the original wording from In-battle effect item was apparently unclear, and it may also be misleading (due to it being out of 256); I tried to reword it, but if anyone is able to word it any clearer, it'd be much appreciated.