Those real time deadlines that take you away from your work is what they're talking about. You leaving for those deadlines means someone else implicitly needs to pick up the slack.

Or at least that is the argument.. but there are a few assumptions here that aren't true. Namely, someone doesn't need to pick up the slack, the work will be there the next day in most cases. Someone else with kids might be around. If your boss is treating you different it's on you to call it out and move on.