9.00719925474099200e+015  <-- That's exact.

9007199254740987.000000
9007199254740988.000000
9007199254740989.000000
9007199254740990.000000
9007199254740991.000000
9007199254740992.000000
9007199254740992.000000   <-- Least significant
9007199254740992.000000       bit dropped.
9007199254740992.000000
9007199254740992.000000