SL that was some damn witchcraft you just pulled lol.
Jeep, what was each of your 60' times since you trapped the same? I would bet yours was better than his. Back when I was stock I ran a turbo 300zx at the track. He was on slicks or radials and nailed a 1.9 60' while I spun and got a crappy 2.4 60'. The whole run he was pulling pretty good and he trapped 98mph. I ended up trapping 99.4mph. The difference is that he had a 13.9 or something et to my 14.8. He was clearly faster even though i trapped higher. This is quite an extreme example but I'll try to get my point across clearer.
Let's say the mustang was spinning more than you off the launch, it would give him more time to pick up speed to trap higher. If you nail a good solid run with minimal spinning there is less speed being picked up since your actually getting your power to the ground.
Its hard to explain, but next time you go to the track if you want to experiment launch really hard and make sure you spin bad. I bet you will run slower but trap at least 2mph higher than most of your runs.