If you are working on a review, are you the only one working on it, or can others be reviewing it at the same time. If so is it then down to whoever gets it done the fastest? Or, once you pick a question to review is it solely yours to review until you either finish or skip? Is there some time-limit in the review process also, I was wondering?
The system is designed to balance reviews getting done in a timely manner and reviewers not getting too often frustrated by reviews that get completed by others while they already invested time in them.
The short is that at least for certain review tasks the task is set aside for one reviewer for some time after they start it (If I understand correctly, this is by now the case for all but close and reopen reviews.) You can read up on details starting on this post on the main meta site:
See in particular the answers by gnat and Shog9.