This is true, had 3 Princess Jellies left and the DRG and I noticed as we where fighting one it was Actually getting more HP then we could take away. Didn't know why but decided to switch to another Jelly.
Needless to say ended up with the Queen Jelly at about 30% HP left. The RDM was nuking it and was slow going, got it down to 1% and noticed it jump to 2% where we timed out.. I think it was getting hit with Thunder the moment it jumped to 2%. We decided no nukes at all after that, It may have somthing to do with the spell it's strong against that will feed these things HP.
Edit: Each one is stong against different elementals, they are not all the same.