I am using a 1.5.2 moodle version, but this bug is still present in the moodle.org Teacher's hands-on playground.
When editing a calculated quiz question, there is a place for an optional unit. However, the indication of the unit does not affect the evaluation of the student's response to the question. this meas that if the numerical part of the answer is correct, the answer is considered correct whatever is present (or absent) as measurement unit.
Moreover, the additional units seems not to be considered at all, since they disappear when the edit question page is reopened.