I spoke to a United employee about this once. I had one of those very rare intl upgrade coupons, and checked ITN before going to the airport. It showed business as about 2/3 full, and I figured I was set. At check-in, I was advised that I was waitlisted for business. I told them what I had seen, and they told me their system showed only one C class seat. According to him, what ITN shows (which is pulled off Apollo) is availability to sell a ticket, which may not correspond to actual seat availability. Made sense to me.

I did end up in Business, so all's well that ends well