"In short, you're mostly correct. We took a "snap shot" of the average game time, took the relevant IP gains, and said, "Okay, this is going to be the IP gain per minute for League of Legends going forward." We constructed a system that would produce those values (plus accounting for time in champion select, loading, etc., so it has a "base" value in addition to the "per-minute" IP).- Phreak, Community Coordinator, via the official forums.
As has been said by others, some users will end up losing IP in the long run because their games were shorter than the average. Some users will end up gaining IP in the long run because they games were longer on average. This is unfortunate for those of you who ended up getting hurt by this system change, so I'm sorry for that.
The reason we are using the new system is to allow for a better balance between maps. Instead of forcing -22% on Treeline for average shorter game times, we have a system that makes everything fair regardless of averages. This is a much cleaner way to go about making all maps accessible without having to constantly research and update multiplier values for IP/XP gains."
More Commentary on the Recent IP changes
Posted on Thursday, March 31, 2011 by Moobeat
Dominatius Maxim created a helpful excel sheet to figure out your IP gain. Phreak replied to the thread giving everyone a little more insight into why the values were changes; it wasn't just an arbitrary choice to mess with us but an overhaul to the actual system to streamline it for future content.