I have actually seen the buy-up prices fluctuate durin successive check-ins on the same platform.
For example, I was offered DEN-IAH for $199 on iPhone app, I then exit and re-enter checkin and the offer is $299, but then drops to $199 again on the 3rd attempt. This is all within minutes of each other. I think this is pretty good evidence of IT issues and not a result of UA's ability to dynamically price upgrades effectively.