I’m convinced that this must be caused by some glitch in Project 2013. I’ve checked that the hours for a resource are distributed evenly (8 hours each day for 2 days). I let Project Level this resource. I changed the predecessor of another task with a different resource – the schedule updates, obviously. When I go back to the resource usage table, the first resource’s task is now spread out over 3 days (6h for the first day, 8 for the second, 2 for the third). When I undo this, the hours revert back the two day 8h per day.
Unfortunately, every time I level a resource I need to check to see that the days are messed up again, but this appears to happen randomly.