Also, non-revs are often not cleared until much of boarding is complete. By that point in the process, if you've already boarded, the only way for you to receive the upgrade would be if the GA upgraded you then came on board to move you up to C+. While this makes sense for F upgrades, it is not worth adding this complication to the boarding process over a marginal difference in product for C+... especially when, as has been mentioned, the person being "upgraded" may even be pissed off about it.

So instead, any remaining C+ seats at this point in the process go to the non-revs who are cleared directly into the best remaining seats available.