It happened to me sometimes too. Also the counter counting down when you place a mine :/ (I would laugh if it actually allowed me to place more mines :D)
I was playing streets of fire and this happened to me. It said I had 14 mines. First, I placed six mines, then I placed another two, but it continued to say 6 mines placed. I placed another, some blew up, and it said FOUR mines placed. I ignored it, and I guess someone triggered one of my remaining mines, then it said 14. I had ammo upgrade, if that matters.
i did NOT take the diffusal skill, that much i know for sure. I had... ammo and i THINK health and speed.
I think this has to do with defusing. I got this bug earlier on CDR canyon. I had just layed about 4 or 5 mines in the NF VF, when i got killed, and someone started defusing my mines. When I spawned, the counter said 1/8, then it changed to 15/8, and then to 13/8. I think defusing takes 2 away from the counter or something.