Being slightly pedantic here, but what's actually happening is that 6,021,001,100,101,010,952,552,448 is the most accurate IEEE754 Double representation - for all of your input serial numbers. The small differences between them are beyond what a Double can distinguish.
For comparison, the next bigger Double with no other values between them would be 6,021,001,100,101,012,026,294,272 - about a billion higher.
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