RPI is basically "record vs. strength of schedule." BPI takes a lot more into account such as scoring margin, pace of play, location, and the like. Here's a fun article for explanation.

http://espn.go.com/mens-college-basketball/story/_/id/7561413/bpi-college-basketball-power-index-explained

When dealing with a metric that is pretty simple (RPI) and you play in a game that is a huge outlier (BYU W vs ZAG) that falls under that metric, you're likely to see a leap. BPI has so many different aspects to it, its less likely to have a large leap or drastic change when only one portion of the metric is affected.

I personally love all these different ways to measure college teams. It's one of the things that makes college sports unique.