Playchess players use different books and different hardware.  Either which by its own or in combination can make an engine play much stronger.  When I tested the beta it was obvious that it was a beast.  With the simple generic .ctg II book and no optimizations whatsoever, my rating climbed to 2860.  Before that it would go around 2750.  The guys on Playchess have tuned books which look for any weakness in the generic books out there and sometimes even with inferior hardware they can score much better than what they should be scoring.  The favorite Rybka on Playchess is 2.1d3 ... I made a 50 game 5 0 blitz match between Rybka 2.1d3 vs Rybka 2.3.1 and the beta won resoundingly +13 -2 =35.  So I wouldn't count it out.  Probably the best way to figure out what the strongest Rybka is, is by seeing what engines the top players on Playchess are using.  Also you should note the time controls ... for example 2.1d3 might have been best at 5 0 but 2.2n2 might be better at longer time controls.