Tyre mounting is a matter of technique... yeah, some rims are slightly larger in diameter than specifications, like a lot of Mavic rims. Combine that with a tyre that's slightly undersized, like a lot of Specialized kevlar-beaded tyres and you end up with a pair that's difficult to install. But it can be done with certain techniques...
1. pump up tube slightly so that it's rounded, but not expanded or inflated
2. stuff into tyre
3. Lay tyre+tube on one side of rim, push valve-stem into hole 1/2-way
4. push tyre down over rim to get one side of tyre completely over edge of rim, leave tube on outside for now.
5. after one bead of tyre installed, push tube into rim, now the remaining tyre-bead should be sitting flush on the braking-surface.
6. pick a point off to the side of valve-stem and push remaining tyre-bead over rim.
7. When you go past the valve-stem, push the stem up into the tyre so that the tube isn't pinched between tyre & rim
8. the last 6" of tyre-bead will be tight, might be easier to deflate tube slightly
9. DO NOT try to push up on the centre of this 6" of remaining tyre, you'll have to stretch the kevlar or steel bead and actually increase the diameter of the tyre-bead in order to stretch it over, not gonna happen
10. instead, push SIDEWAYS on the tyre-bead at either point where it rides over the rim-edge. This spot is [i]already stretched over the rim, so you don't have to stretch it, just push it sideways. Gradually work the two spots closer and closer together until the whole tyre is installed... remember, the secret is to push sideways on the tyre, not up & over.
11. pull the edge of the tyre sideways away from the rim-edge so you can see down into the rim, no tube should be visible. Work your way around the entire rim to make sure that the tube isn't pinched. Then repeat for other side of rim.
12. pump up tyre with 2-3 pumps, spin wheel and check for even tyre-centering and no hops or dips. Adjust tyre by pulling up on low-spots as necessary. Then pump up to final inflation pressure.