*Dons the maths hat* (and yes, it is Maths, not Math, silly foreign types).....
I think you'll find, according to my indepth caluclations, that 264 - 1 does not in fact equal 18,446,744,073,551,615.00 but 263.
On account 263 is 264 - 1.
Unless you mean 264 to the power of one, which would be 264......