Surely one reason why a US airline might publish precise timings is the monitoring of delay times and their publication.
If I'm going to be named and shamed if I'm more than 5 mins late, I'm certainly not going to round off the times to the nearest 5 mins, as I'm using up the margin. A flight expected at 7.06 is still 'on time' at 7.11, whereas if declared at 7.05 it would now be 'late'.