Buying a champion with IP.

Last night I wanted to check which champion to buy next since I had 3120 IP and was getting close to the 3150 IP champions. I then realized it said that I could buy one of the 3150 IP champions and I was confused. So I tested this out and got Shaco. I then got 30 IP left over (which is the difference). Just wanted to bring this up to you guys that I got a champion for 60 IP less than I should have.