I have not read where the mother says she doesn't remember what Caleigh was wearing that day. I did see where she was not positive that the remnant of the shorts they found belonged to Caleigh. I can understand that, they were in tatters and not recognizable.
As far as the time, it's hard to judge time when something like this has happened, I think, but an incident like this could happen in one minute. Try it sometime... look at a clock with a minute hand and count off the seconds to a full minute. It will seem a lot longer than you think it would.
However, I agree that it doesn't make sense in a lot of ways. This woman seems to be in denial that she did anything wrong, but IMO, and in a lot of others', she should have forgot the ball, and never left her children unattended. Two small children near water with no one to watch them every second, it's a recipe for disaster.
Also, would like to point out. None of these cases ever "make sense." If they did, we wouldn't be here talking about them.