a couple of reasons
First, you generally do have a slight edge @+1. Second, you want to minimize the amount of time you are standing behind the table watching as everyone knows exactly what you're doing. When backcounting I come in on any positive RC and walk on any negative RC, hence, I only backcount one round. I'm not advising this, but that's what I do. Only very small bettors can stand behind a table watching for long periods of time and then start plopping down bets, only to switch to another table shortly after and do the same thing.
Another reason, in general, to bet bigger at +1 than at 0 lesser counts is so that you won't have as big of a bet jump if it goes up higher.