Most computers use base 2 for floating-point numbers as well as for integers, and
just as for base 10, not all fractions are representable exactly in base 2. It's
well-known that in base 10, a fraction like 1/3 = 0.333333... repeats infinitely.
It turns out that in base 2, one tenth is also an infinitely-repeating fraction
(0.0001100110011...), so exact decimal fractions such as 3.1 cannot be represented
exactly in binary. Depending on how carefully your compiler's binary/decimal
conversion routines (such as those used by printf) have been written, you may see
discrepancies when numbers not exactly representable in base 2 are assigned or read
in and then printed (i.e. converted from base 10 to base 2 and back again).