The thing is, there are too many variables affecting how long combat takes to resolve that have nothing to do with the game’s design. However, from close analysis of the combat math, it is clear that a medium to hard encounter should take about 3 rounds. I think this is the result of that 10-minute combat goal. They probably estimated that a round of combat would take about 3-5 minutes on average and shot for 3 round combats to be the norm, figuring that would put them within acceptable tolerance of that goal.
