I heard that, while it's not first come first serve, you would have a higher chance getting a place if you applied earlier than someone who sends their application in, say, December.
The university will receive early candidates first, and evaluate them. They'll offer places to the people they want to offer places to!
The grade requirement may be AAA - they have loads of places free, since applications just opened.
As time progressed, however, more places are offered - there are less free spaces. Now, the university may decide to be picky about who they select. They'll want to choose the very best from the remaining candidates. The offer may now be an A*AA.
I'm not sure whether this is true, but it does certainly make sense. Fewer places towards the end mean that the standards will get set higher.