My guess would be that they use different pieces of each software program. Asterisk is good for some things but FreeSwitch can be used as a SBC (session border controller) and from there, you can do load balancing across multiple machines. etc. as an example.
I think you pretty much nailed it. I know a few guys that run a VoIP carrier out of Cleveland, OH. They started out building their core SIP switches on Asterisk, but quickly learned that Asterisk isn't the best piece of software for that role. They moved their core to OpenSIPS, but still use Asterisk for feature offerings (like conference bridges and IVRs).
