Basically, it is this:

IEEE designates that doubles should be held as 64-bits in memory. The Intel FPU, however, has 80 bits available for calculations. So it comes down to 1) being aware of the platform on which you are running, and 2) using a programming strategy that ensures the FPU is used as you need it to be used.

For many applications, the extra 16 bits of precision is a good thing. For some applications, such as mine, a much greater control over rounding and precision is needed, and means are provided for making use of what you need to use.