I am first hand when it comes to FIV in kittens,. I rescued a momma that was FIV and she had 3 kittens. 2 POS and 1 NEG. I was just distraught. BUT as you said it is VERY RARE for them to get this from the momma, I continued to test monthly and then one day 1 tested NEG, and the following month the last one also turned NEG. So yes it was pretty well 6 and 8 months before this happened. BUT IT DID HAPPEN!!!!!!!!!!!
So yes it was probably a test that turned positive from the mothers antibioties, which show up for along time. Keep testing you may just find out that the kitty is TRULY not POS.
In Fact VERY LIKELY not POS

Carrie