this seems like a floating point moment. probably, anything greater than that number you have found will actually be represented as 550, and that and anything smaller than it will be represented as some number slightly smaller than 550. if you wrote the numbers in binary, it'd be the difference between 1000100101.1111...110 and 1000100101.1111...111. i think.
i assume that's what's going on anyway. it might be something completely unrelated