I guess they discourage you for playing too many games in one sitting
So there are several things:
- You get less IP the higher your level gets ( slowly ramping up from level 1 to 30)
- Every map has min/max caps on IP earned
- Summoner's Rift: 18 IP + 2.312 per minute IP for a win ; 16 IP + 1.405 per minute IP for a loss
- There is a first win of the day system (+150 IP for the first win)
Hope that clears things up a little bit.
Also, I think it's worth mentioning that they do not want to reward players based on personal score. They don't want to promote scorewhoring and killstealing, only the overall outcom of the game is supposed to matter. Which in this case is based on if you win or lose. Personally, I would enjoy if teams who won quickly got rewarded for it. Since there are a lot of cool blitzkrieg/rush strategies you can pull off. Dragging games out to gain more IP isn't a cool mechanic. :/