If you did a SDC you shouldn't be on the standby list. Sounds like you did a SDS.

SDC = same day confirmed
SDS = same day standby

Lots of confirmed passengers will appear on the standby list, it's basically a list of people without seat assignments, not an actual standby list. Confirmed passengers without seat assignments will be ahead of actual standby passengers.

Generally, the priority will be
Confirmed pax
Revenue pax on standby
Non-revs

I have cleared into C+ on occasion from SDS, but not always. The GA will process all the upgrades first, then standbys, so generally there are no seats left to clear into.