Alien Registration number

[Krdaya]

Hello,

Does anybody know under what circumstances USCIS changes a non-immigrant's Alien Registration number? When I first got an EAD after graduation, USCIS assigned me an alien number. 2 years later when I applied for an EAD under different conditions, USCIS assigned me a new alien number.

I thought alien number is like SSN and cannot change. Any ideas why it happened to me?

Thanks in advance.