Paternity testing question: what's all this "probability" nonsense?

The stupid paternity lady that I'm on the phone with is pissing me off because she won't answer my questions, and she doesn't know what the hell she's talking about.
Here is the situation:
Woman A is 100% known to have Mother A and 100% known to have Father A
Woman B is 100% known to have Mother B and *questionably* Father A
IF Woman A and Woman B both had Father A, they would be half-sisters. I was told by the paternity test people that if Woman A and Woman B were both tested, there would be an "86% probability that they are half-sisters." I kept asking, "does that mean 86% ACCURACY?," to which she replied "no, probability." Does it mean that they will share 86% of Father A's chromosomes (86% of the 23 chromosomes that he passed on to each of them)?
Then she keeps saying that it'll come back 86% if they ARE half-sisters and 0% if they are UNRELATED. 86% what?!!?!?! She then offers to have Mother A and Mother B tested to raise the "probability" to 90%. Probability of WHAT? Thnx!

I guess tests don't lie, either knowingly or unknowingly.