yabs plays only with opponents rated under 1700. Its sister, yabe, is the snooty sibling and plays only with opponents rated over 1700. They seem to be the same program, which begs the question: "will their ratings differ, given that the FIBS rating system should ensure they stay roughly equal?".
Good question, and I'm glad you asked. See the list of bots sorted by rating to check their ratings last time the list was updated. Currently (30 Jan 98) it seems that yabs (playing low raters) is 100 points higher than yabe (playing high raters). Does this show that FIBS' rating systems is skewed? Perhaps, it's not for me to say (I become confused when square roots come along). It is known, though, that playing 2pt matches against higher rated players benefits you, and one_pointer (Bernd Kaiser), a strong player, is consistently in the top ten on FIBS by playing only one-point matches. See the Backgammon Galore newsgroup archives and Kevin Bastian's explanation of FIBS ratings for more discussion on this matter.