As far as the MTB wheels: I would bet that those MTB wheelsets all have 20mm front hubs, which don't have axles (the axles for 20mm hubs are proprietary to the fork). Also, disc rims are usually lighter than rim-brake rims too, since they don't need a braking surface. Tubeless rims are even heavier than those too. Add in the fact that the rims are a lot smaller diameter-wise, and you get a not-too heavy wheelset.

Also, Mavics are about the lightest trail bike/freeride wheels you can get. Here's a real-life comparison of a few freeride wheelsets (from a thread on ridemonkey):

Azonic Outlaws
Front: 1118 Grams
Rear: 1328 Grams

Mavic DEEMAX
Front: 1120 Grams
Rear: 1330 Grams

DT Swiss FR 2350
Front: 1070 Grams
Rear: 1180 Grams

Sun/Ringle Single Track
Front: 1100 Grams
Rear: 1220 Grams