The aircraft is flying, right, just consistently behind schedule? So I would guess that it required about five hours for maintenance but has quick turn times so there's no opportunity to catch up with the schedule. It's not like it's been in maintenance for five days unless DL is swapping the aircraft rotation around or doing different turns--i.e, the aircraft for ATL-NRT used to be the XXX-ATL bird, but now it's YYY-ATL. I had understood that it's a single tail number that's been late.