Buying Champions

Ohh don't even know whether It's a bug or something else.. I will just explain what happened.

So I had 6506 Influence Points.. And bought Twitch and Miss Fortune in quick succession.

But after leaving shop I noticed that I still have 3356 IP. ( It means one champion cost me 0 IP)

Plus after all this , I restarted my client and both champions were owned with only 3150 IP spent. I also was able to pick them both in champion select.

That's just It.