Game Center sandbox mode? Not jailbreaked.

[Dirtysand]

My device is NOT jail breaked and runs on iOS 7.1. Today I reset my router firewall which had previously been redirecting connections to Apple to a local machine for testing purposes, and after I opened up the Game Center app, I noticed that it wanted me to create a new account, I have never encountered this error before on a no jail broken device, so I complied, and it showed that it was in sandbox mode. The only app that had any issues before the firewall reset was Asphalt 8 by gameloft, which showed my Game Center user name to be '(null)', all other games had worked fine. This was why I reset the firewall. Didn't really lose any data, but I'm still curious as to why this happened.