Mathematica interprets the real numbers 9999999999999999.0 and 9999999999999998.0 as having machine precision. To work in arbitrary precision, you need a backti

Mathematica interprets the real numbers 9999999999999999.0 and 9999999999999998.0 as having machine precision. To work in arbitrary precision, you need a backtick after the number, followed by the number of significant digits. In this case, 9999999999999999.0`17-9999999999999998.0`17 does indeed return 1. https://reference.wolfram.com/language/ref/Precision.html https://reference.wolfram.com/language/ref/Precision.html