AFAIK, and I could be wrong, the one for SF is not just purchasing credits, but electricity from local power suppliers. I believe the way it works here is a certain percentage of electricity is supplied by the renewable energy partners and PG&E is responsible for the remaining percentage. This percentage is based on the number of people who have elected to go the green power route. Obviously they can't do this on the fly due to how power generation and grids work, but on a month long scale they have more than enough time to modulate generation sources to get close enough.

So let's say 60% of people are signed up for the program. They'll get roughly 60% of the city's power from the renewable sources they have, and allow PG&E to generate the rest. Sure the transmission of power to my place isn't going to be 100% renewable, but that doesn't matter as someone who is on the non-renewable plan somewhere will receive the renewable generated power anyway, so the net effect is the same, and I still get cheaper electricity generation charges.