I cant give you a full explaination, because I don't know myself. however, when you total time up excel goes upto 23:59 then onto the next day by default, so if you add two minutes to 23:59 excel would come back with 00:01. By using the [h] format it just keeps totaling up. Maybe one of the gurus out there can explain better, I will be keeping an eye on this post for a better answer.
Cheer's
Colin.