offline bounce different than realtime bounce

[grant howarth]

Hi all,
I have just done my first mix in protools 11. I did a real time bounce down as i normally would and then i figured i would give the offline bounce a try. When i brought both files back into protools i flipped the phase on one of them to see if i could hear any differences. (i believe this is called a null test).
I was surprised to hear some guitar during the playback. Specifically the tracks that I used the "waves CLA Guitar" plugin on. When i removed this plugin and did the same null test the guitar disappeared.
With a careful blind listen to both mixes i was able to pick out the mix that had the off line bounce as the guitars in it sounded just slightly quieter.

So i guess i am wondering, is this a known issue with protools 11? Would this be a bug in the waves plugins? Any info would be greatly appreciated. Thanks!