It's not the same robot. I remember distinctly. The two don't even exist in the same continuity. X was a reploid that was programmed to emote and to think individually unlike other robots. He was held in stasis for a number of years, and was discovered eventually. I can't remember if the other reploids were copies of X or if they already existed by the time he was reactivated. One way or another, it's not the same robot. His name isn't even technically Mega Man, his name is X.