Often, the standbys are Delta employees flying NRSA (non-revenue). They would get the open seats in C+ (non-medallion paying pax are in the back unless the flight is oversold or are in an IRROPS situation). This is why you can't count on a middle seat in C+ going out empty; at the last moment, DL will give that seat to an NRSA standby who wasn't on any gate display standby list).

Interestingly, some DL gate agents don't get around to upgrading medallions from main cabin to C+, and that such pax who want the upgrade to C+ have to go up to the gate agent and ask for it.