Sorting differences

I generally sort the differences lists from my SmartSync profiles by action, descending (arrow pointing downwards on the action column). This puts collisions at the top of the list, if there are any, followed by deletions. I do this because these are the actions that I'm most likely to want to check before going ahead.

However when the list is very long (I'm not sure how long the list has to be to qualify as 'very') the sort works in reverse; the arrow points downwards as always, but now collisions are at the bottom, deletions above them, and so on.

Am I misunderstanding something, or is it a bug?