The meter is only for the mob your currently are targeting. It's best used to get an idea of if it's OK to continue ignoring a target or not based on how far ahead you are from everyone else.
I would say that it's not impossible to get a mob back when it's near dead, but a lot of experienced DD's will decide to go full out on a mob to kill it off. Because of this, there is little hope (or reason) to even attempt it.
As long as it's not the healer that's pulling aggro, it doesn't really sound like there is much of an issue with what your experiencing. It sounds like a normal over geared and over zealous DD wanting to do as much as they can within the run.
I believe that is the region of leveling right before the RoH ability where DD's can and will pull of if they want to. Once you pick up the third rotation ability then you'll see those issues disappear.