This won't explain exactly why it's happening, but here's some random information on what may be occurring somewhere in the website.
It looks line a numerical overflow somewhere by the website server.
http://www.binaryconvert.com/ will show you that the maximum possible value for an (unsigned) integer comprised of 32 bits is: 4,294,967,295
This is when all 32 bits are set to '1' (http://goo.gl/Xwysf).
Unsigned means it can't be 'signed' as either positive or negative; it's always deemed positive.
A signed number however is flagged to be either positive or negative by using the first bit of the 32 available. 0 means positive, 1 means negative. Because you're sacrificing that first bit, you have a power-of-2 less available numbers to represent in each direction (2147483647 or -2147483648)
When you subtract a larger 'unsigned' number from a smaller one (which would usually result in a negative number), it rolls back to the maximum number again and continues to subtract from it.
What's particularly odd though is that the number you're seeing is 569 higher than the maximum possible unsigned integer with 32 bits - probably the result of mixing 32 bit numbers with 64 bit numbers somewhere along the line.
64 bits means you have 32 more '1's or '0's with which to represent a number...so you can represent absolutely huge values with it (18,446,744,073,709,551,615 to be precise).
This information will, of course, have helped you in no way whatsoever regarding your stats issue.