Did I hear somewhere that RideLondon is organised by the same people as the London Marathon? If so, there was a website going around a running forum sort of describing how the so called ballot works.
The organisers want a steady flow of people through the finish rather than a large bunch at one time. So based on the theory that you will have some quick people, some slow people and a whole bunch of people in the middle, you can have a rough guess that you will have more chance in the ballot of your quick or slow. Then add in the fact that you have to have a mix of gender and age groups, so if your Joe Average, you've got less of a chance than if you put a slow time because your doing it for a bet.
It may be different for this and if it is the same organisers, hopefully that may give some insight into "ballot" systems.
If I ever find the link, I'll post it.