My in-laws never fail to show up 15-20 minutes early wherever we go, or worse, to our house, when we as hectic parents are trying to use every last second to clean up and then we're left awkwardly still getting ready while they're waiting in the living room.
Then they'll tell us to come to their house at a certain time, say "around" 6PM, and they'll call at 5:59 asking where we are. I admit, we're 5-minute-late kind of people... and I do take my liberties with the word "around" but you'd think sheld at least wait until 6:05PM.
Of course this is just a nuisance, not an actual problem that's worth getting into a discussion over, but it irritates me to no end... so I kind of see where the OP is coming from getting so annoyed over it. But in my opinion it's worse when you show up to a restaurant they've been at for 20 minutes and they haven't even ordered drinks because they've been "waiting" for you.