Sounds to me like the axle nuts are the correct size, but incorrect thread pitch and somebody just forced them on. They would seem tight because they were cross threaded. When you took them off, the mashed up threads wouldn't hold anymore.
edit to add: The good news is that the axle is case hardened and the nuts probably aren't. So the axle threads should have survived the ham fisted repair job.
Last edited by BigChief; 01-11-16 at 08:58 AM.