don't see your logic. the reason the arb is there is because one book (probably the other one) has odds out of line with the true probability of the event happening. Therefore you would expect to win more bets at the out of line book and thus lose more at Pinny. if you can stand the variance, don't bet the hedge at pinny and clean up at the other books!!
I've seen it happen loads of times, if I hadn't matched/hedges most of my bets at a low juice book (eg pinny), i'd be a millionaire by now
If you're betting -110 lines at the other book, which sees the line as even money probablity, you need at least +111 at pinny to make an arb so the difference in real probability (if you pinny is right) is more than the 2.5% you state.