The number 9999999999999999 is just above the precision limit of double-precision floating-point. In other words, the 53-bit mantissa is not able to hold 9999999999999999.
So the result is that it is rounded to the nearest double-precision value - which is 10000000000000000.
9999999999999999 = 0x2386f26fc0ffff // 54 significant bits needed
10000000000000000 = 0x2386f26fc10000 // 38 significant bits needed
