I had a bit of a dilema on that point. I'm reasonably connected with the london startup scene, which naturally has connections to the YC scene, so I know through a fairly reliable grapevine that some of the acquisitions were "mercy acquisitions". However given this information is essentially "private" I don't feel comfortable publically disclosing it, so I've stuck to public published data about acquisitions.
I'm going to go into the acquistions a bit more in another post, I'll see if I can find some way around the dilema.
I'm sure for every round of YC, there are a few companies that don't even make it to launch. The ones that don't launch are not even listed.
Also, a vast majority of the acquisitions shown here were for talent. They generally involve very little actual money and in many cases could be considered company failures.
Still, I'm happy for the successes. But in startupland there are many magic tricks to make things look better than they are.