Mmm that's weird,because on my iPad those black bars don't show up,but my controls and side buttons are so small,they aren't as big like the ones on your picture.
I'm pretty sure it is lag; it used to happen for me maybe a year or half a year ago, before pinch-to-zoom. It would usually fix itself within a few seconds though.
There's also a bug when going on shrink mode on iPad 2,controls show all messed up and tiny and zoom control option doesn't help at all,it just centers the controls but it doesn't enlarge them.
I'm pretty sure it is lag; it used to happen for me maybe a year or half a year ago, before pinch-to-zoom. It would usually fix itself within a few seconds though.
The black bars stay there until you walk away and can't see them anymore but then another one shows up...