OT: Why do different sites have different RPI calculations?

So I've wondered this for a while, but it seems that every site that does RPI calculations for basketball ends up with different rankings/values for every team. I thought that the RPI formula was a well-defined calculation? I could possibly understand a little variance, especially early in the season: initial ratings bias, whether or not all games played count (some teams might play games outside of D-I, which may or may not be counted), etc. But for instance currently ESPN RPI has Ole Miss at 33, and another site that I check, rpiforecast.com has them around 121.

Is it just early season inaccuracy that will eventually level out, or are there additional factors that each statistician can play with for their own rankings? I feel like I've seen sites disagree by more than 10 spots late into the season in the past, which is significant considering how much of a factor RPI can be on bubble teams and NCAA tourney seeding.

Wikipedia link regarding RPI for reference.