This could be any of a huge number of things.  You should post the games of which you notice this to be the case, and people like Vas, Larry, and many others on this forum would enjoy taking a look at them.  It's possible that Rybka simply misevaluated the position to begin with, and that it was really always drawn; it's possible that there is a tablebase error, it's possible that Rybka is simply choosing bad moves in the endgame, etc.