Did your druids die? If they do and re-enter to come back to life that counts as a new instance for them.
I can't answer the question, but I can say that this is wrong It would be a bit inconvenient having to clear the whole instance again every time you wipe on the last boss!

I don't think there can even be a bug here that would make this true for the purposes of the reset limt. Both boosting and not boosting, my guys (and gals!) have died far more times than the instance reset limit would allow if this were true.