Did you have any other legs for that day? Min credit sets the min for the day but if your credit already exceeded :15 then it would default to the higher amount. for example if you flew 1:00 and then did the taxi for :10 the min credit for the day would be :15 which was exceeded by the two pay segments. In this case set your scheduled taxi time for :15 and the actual for :10.
That is interesting. I assumed min credit for a leg would be just that. Maybe we should have a min daily credit under airline settings (although setting block to this is a good workaround) as well as payroll category settings for leg min credit and also one for leg "specific credit" (i.e. you could setup "training" to always credit as 4 hours, regardless of what actual times were entered.