Riot Store overcharging IP on champion purchase

I just bought Kog'Maw with IP from the store. Prior to buying, I had in excess of 20000 IP (although I don't remember the exact number).

After buying, I only have 11468. Since Kog'Maw only costs 4800 IP, this would indicate I started out with 16268 IP, which is much less than I had.

I'm not sure if I was doubled charged, but I would appreciate if someone could look into this.