3.3.4.1 Why Are My Floating-point Results Not Quite What I Am Expecting?

Since floating-point variables only have a finite number of bits to represent the values they are assigned, they only hold an approximation of their assigned value (see Floating-Point Data Types). A floating-point variable can only hold one of a set of discrete real number values. If you attempt to assign a value that is not in this set, it is rounded to the nearest value. The more bits used by the mantissa in the floating-point variable, the more values can be exactly represented in the set, and the average error due to the rounding is reduced.

Whenever floating-point arithmetic is performed, rounding also occurs. This can also lead to results that do not appear to be correct.