The way the test works, theoretically, is to detect male dna.  If male dna is present, then the baby must be a male, because it can't come from the mother.  If no male dna is found, the baby must be a girl.   So the test doesn't try to detect female dna.  There is no simple way to distinguish the baby's female dna from the mother's female dna.  

Oh...they are so full of it!  Let me tell you what I did when taking the test.  I had washed all my towels and clothes the night before.  I wiped down the bathroom with clorox.  did the test at 5am when everyone was asleep!  Washed my hands 10 time....super, super cautious!  There is no WAY I contaminated my result...NO WAY!!  That is just an excuse they use for their test not working!