Looking at the controller settings on the PC while moving the left analogue stick around I found I was pushing slightly 11 o'clock when I thought I was pushing dead straight (as per prior assumptions I was doing this from other gaming on the Playstation). After a lot of concentration I can get a pretty accurate perfectly straight push, but it is bloody hard. I'd check this if to make sure what you think you are doing on the controller is actually what you're doing.
Note - Looking at the controller as you're doing it is not the same as if you are looking at the PC/TV screen while doing it. You will always be able to correct any errors if you actually watch the controller as opposed to watching a game where you have no finite instant feedback on stick control.
