Sounds like it was the ambient temp. From 93 to 100 makes a big difference in energy lost in the A/C system. Do also check to see if they just switched to 10% ethanol. It will give you a mileage hit (but it will also burn cleaner). You can smell the ethanol so that's easy to check.

 

For me, in the summer I can expect 41-46 at 78 with the AC on. I've tried slowing to 65 and it doesn't make much difference. I've also tried running without the AC, and that isn't worth the sweat stains and misery. It is what it is, and it's way better than most other vehicle out there by a considerable margin. Going over the Big Horn mountains to Yellowstone and Jackson, Wyoming, twice this summer I've averaged over 54 round trip for both trips. From Jackson to Cody, WY, I got 57 both times (speed limit through Yellowstone is 45 but there is a lot of climbing and high altitude). Normal driving around Sheridan, WY (15,000 population) in the summer I get 52-55 typically with no special effort other than easy acceleration and such (no pulse and glide). I have noticed that my 52-55 will drop to 45-48 if I use the AC around town (lots of stop signs and stop lights).