LP Miscalculations ?

So my brother and I was duo ranking and he has 85 LP we won a game which gave him +15 LP this game was supposed to send him into his series but, his total LP came out as 97 LP ....

"85+15=100"

Exactly Vel, so how does 85LP+15LP=97LP either ranked system is bad at math or there is something we are missing.