One of the things that is not taken into account with the C rating is the batteries internal resistance. This may get a little deep but I'll try anyway.

Every battery has an internal resistance, when you draw current out of the battery the voltage drops besause of this internal resistance. A battery with a high C rating like SlotCars 14C batteries have a very low internal resistance. So when you draw 14C worth of current you still get 7volts (an estimate) with a charged 2S pack.

Batteries with low C ratings like 8C and some of the cheap LiPo's have a higher internal resistance, which when loaded to 8C may only give you 6.6volts (an estimate) with a charged 2S pack.

The lower voltage reduces the motor speed, so you give it more throttle and the battery runs flat sooner. The internal resistance also creates heat in the battery, which is why you shouldn't discharge a LiPo at greater than its recomended maximum or you'll get to see for real what all the fuss is about (boom).