I've had a number of older milsurps that shoot low. I suspect that a lot of commercial ammo is loaded nowhere near as hot as the original military ball for safety reasons - the ammo manufacturers have no idea how old the gun that's going to be firing the ammo is, so they load it light to be on the safe side.
Fortunately, it's easy enough to deal with. Just push the rear tangent up to a longer range, one click at a time, until you end up "zeroed" at 100 meters (or close enough). Saves you from having to do any permanent mods, and you can quickly go back and forth on your range setting for different ammo.