The default format for a student's answer is a Formula MathObject, not a Fraction MathObject. Hence, WeBWorK gets 0.2, not 1/5, in the standard answer checker. So the ultimate issue is the parser that interprets the student's answer, and the answer is as simple (and as complicated) as finding the proper parameter and what its value should be.

In 2014, Paul Pearson mentioned something along similar lines: