As a guy reflective on what our sex (uh, meaning the MALE side of the equation) is like, it might have to do with a few different things:
Possibly he has better technique on the hills?
- Seated riding is MUCH more efficient than out-of-the-seat riding. You go through your energy reserves much faster when you're up off the seat.
- The line he takes when riding up, if it's a technical trail, may be much more efficient/easier than the one you choose, meaning he uses less energy
- Again, if it's a technical uphill, the little riding techniques like popping the front wheel over obstacles so there's no collision/resistance, and getting your butt off the seat at the exact right moments (like when your rear wheel hits a bump/resistance) make a difference too.
If you get these techniques wired, you'll find your hill climbing will get a lot easier, faster, and you'll have much more energy left over after the climb. You might also get the competitive edge on your riding partners as well!
The other reason why he may always beat you and seem like he's done it easier is purely out of male ego. Many guys hate to be beaten by other people, and ESPECIALLY women in particular! So it can't be just being faster than you, but it also means trying to look like it "ain't no thang" either, which means he may be exhausted, but trying to control breathing and such so he doesn't show it.
And, as said before, guys generally have more "power" than girls. Olympics show this pretty well. The average scores/times between men and women always differ. If they were the same, then most events would be "mixed".
Now, endurance and agility are different matters entirely!