So far they have attributed the difference in range to the better aero of the R1S (presumably ranges have been quoted with otherwise comparably equipped vehicles). However, it seems common to lose about 3% ICE fuel efficiency going from AS to AT. According to this article, it seems the biggest differentiator for AT is the higher carbon black tread compound vs higher silica compound of AS. This makes the AT tire more durable over gravel and rock that may otherwise cut/tear/chunk an AS tire. The AS tire will generally be more efficient, ride better, be quieter and give better performance than the AT tire but is not likely to hold up as well as AT off pavement. And to skyote's point, AT also isn't typically your best choice if you're serious about off-roading.I kind of wonder if that's where the difference in range is between the R1T @ 400 miles and the R1S @ 410. For a 10 mile loss I'll take the AT's on 19s or 20s.
My R1S will be more "dog hauler" than "rock crawler" so AS for me without a doubt.