As
UA-NYC had already mentioned,
UA allocates Availability in discounted Booking Classes at its sole discretion.
See also:
Thanks all for the quick replies! I had a similar situation last year and figured this route must be pretty lucrative for UA in premium classes
So the question might be: What comes first? Availability of discounted biz fares or sell out of inventory? I have the odd feeling I have to pony up for this one.... right now they sell C fares only.