Maybe it has to do with the number of steps combined with the time it took to get to that number? Like 2.5 miles over the course of the day means you're generally more active over all. Where as 3.5 miles in...what...45 minutes to an hour concentrates your exertion to a very short time frame and not constant activity throughout the day, so the fuel points are less even though the distance was longer.
Did that make any sense? Apparently I'm not communicating well today.