The frac command computes a/b and reduces it to lowest terms. For example, the fractions "4/8" and "2/6" are presented as 1/2 and 1/3, in simplest form.

This is done automatically in Maple when you compute a/b, so this function is purely to complete coverage of the Maple T.A. built-in functions.  To prevent this from happening in Maple consider using the InertForm package.

$\mathrm{MapleTA}:-\mathrm{Builtin}:-\mathrm{frac}\left(4\,8\right)$

$\frac{1}{2}$

The MapleTA[Builtin][frac] command was introduced in Maple 18.

For more information on Maple 18 changes, see Updates in Maple 18.