The infamous integer overflow strikes again! That -2 billion XP is what happens when you're so good at gaming that you broke the 32-bit integer limit (2,147,483,647) and wrapped around to negative territory. It's basically the digital equivalent of being so awesome that the universe penalizes you for it.
Same energy as when your bank account shows "-$0.17" but you swear you should be a millionaire. The programmer who didn't use unsigned integers or 64-bit values is probably somewhere crying into their coffee right now.