I was curious about this wiping effect too. The full screen gets wiped
twice and then this narrower one slide up.
When I installed my UM I had an old 3:4 screen (my previous main station
still had the better monitor) and I thought this may have been a
hangover from the smaller screen dimensions.
It does appear incongruous. On my MacBookPro UM installation there is
no grub menu at all. It looks a lot slicker!! Any ideas???