PER season averages are based on playing good teams and bad. In the playoffs you're only facing good teams, and the Celtics so far have faced two good defensive teams.
Not only that but PER doesn't really sum up team wins very well. Its an offensive statistic, you can actually increase your PER by shooting more with a breakeven of just .30 2P FG% and .21 3P FG%.
Rondo EWA 6.4
Ray EWA 3.4
Paul EWA 9.5
Kevin EWA 8.3
Bass EWA 2.5
Avery EWA 0.6
Greg EWA 1.1
Peaches EWA -0.9
Sasha EWA -1.0
O'Neal EWA -0.3
Wilcox EWA 0.2
Dooling EWA -1.3
Marquis EWA -0.6
Total = 27.9 EWA, Actual Wins = 39 So we had 11 more wins
That's our top 13 in MP I believe: only Hollins/Moore/JJJ/Sean Williams got left out for time's sake (and Hollins played for the Cavs making his EWA split)
PER = offensive based superstar comparison stat that overvalues how much you shoot
EWA = garbage.