My biggest worry was that it would be tested to determine if it was real pee. Imagine my surprise when I got a call back telling me that I tested positive for THC!!!

Can someone tell me how this can be???

Only two ways:

a) An untrained tech had misinterpreted a "faint" line as a positive and ended it there with no confirmation test performed (contestable under Federal and State laws).

b) The product had been tampered with (ie: dirty urine added to the synthetic, or synthetic was entirely replaced with dirty urine).

Either way, you've gotten the shaft - thats is, if your story is indeed true.