Almost none of the so-called 60, 70, 80mm fans are exactly on the 10s as they are really built to english standards (inches) 80mm is really 3.125 (3 1/8)" = 79.375mm, 92mm is really 3.625 (3 5/8)" = 92.075mm, etc. It's just convenient to call them on the tens.
.bh.
:moon: