People don't show for the flight.

As far as the airline knows, everyone with a ticket/reservation will show up, so only what's left is shown as available. As people cancel, no-show, mis-connect, move-up, etc., seats free up. I have seen the number climb while watching it at the gate, but it's only for a minute or two as the GA's then start giving the seats to stand-bys and the number again drops.

Flying to CLT, you're probably seeing a lot of what i see flying to PIT - pmUS employees flying to/from home. It appears to me that they can get on the stand-by list way in advance, and just put their name on everything. Last week, my aircraft held 120-ish and there were 64 on the stand-by list. There are typically 40-50 every Thursday/Friday. Almost everyone that clears stand-by is in uniform. I once arrive very late for an earlier flight - the GA wouldn't even put me on the list, she just wrote my name down. I followed along as she called all 26 names on the list, with about 7-8 boarding, and then called me. Again, I suspect all the employees stand-by on every flight so many are already gone on earlier flights.