User blog comment:AokijiSencho/Chapter number v.s. Episode number/@comment-24850639-20140629162114

The anime will never "catch up" with the manga (unless Oda takes a SERIOUS break of course), and honestly you don't want them to. One thing that I've noticed, at least recently, is that the anime episodes always tend to end on those sort of "cliffhanger" moments like they do in the manga. So going along with that, you definitely are on to SOMETHING with the 1 episode per 1 manga theory.

Like 12th said, they tend to add things so that it makes an actual episode since most people can honestly get through a manga chapter in like 10-15 mins. They extended Luffy and Chinjao's fight as a recent example. In the manga their haoshoku haki clash doesn't last as long, nor is it as emphasized there as it is in the anime.