This looks confusing if you consider YC to be an accelerator or incubator, labels which they hate. If you look at them as a seed stage investor, their shifting focus makes sense.
They give $120k for 7% of the company. Since the deal is set, the only variable, to maximize shareholder return, is the companies they accept. Since they are an investor, they're going to pick the best investments.
How do they know the "best" investment? Past history helps, so (relatively) later stage companies will have an advantage. Revenue, team, traction, etc. are all good indicators.
If you look at YC as just a seed stage VC, their actions are pretty consistent and logical.