Well, it actually is a fraction. 1/2 is the same as 1 divided by 2, or .5. 1/3 is one divided by three, or .333. So 1/2.88 is one divided by 2.88, or .3472 of an inch.

The weirdness comes from a decimal in part of the "fraction" - somehow mentally I think we expect one or the other, i.e. .3472" pr 1/3", not a hybrid of the two...