Why iPhone 4S serial number is being replaced?!!

Like the title stated above, I'm currently having an serious issue with my iPhone 4S. I live in Indonesia, and I bought Apple iPhone 4S immediately when it launched in S'pore through my friend who resided there. The phone is unlocked, and up until today I have no significant issues that need my attention. I have yet to check my serial number because I thought it wasn't necessary, but today I just got some knack to checked it and look what I found was this error message from the Apple Support Website.

"We're sorry, but this is a serial number for a product that has been replaced. Please check your information and re-enter your serial number. If your information is correct, you may need to contact us."

I'm pretty sure that I should contact Apple by phone directly to inquire about such issue, however my curiousity is building as I couldn't find the same issue in the community discussion in Apple Support Communities. I managed to find similar issue such as I am, only to found out that "their" serial numbers were not valid or "invalid", instead of "replaced" like mine. So here I am, curiously and significantly dissapointed at Apple why would the serial number has been stated as replaced... Is there anyone out there that could shed some light into my issue? Thanks and appreciated it.