The numbers aren't meaningless, but yes there is a high degree of change to be expected. If theres 20 showing available and already 18 listed above me. I wouldn't be relying on that flight.
If people have used their free firm ticket, then they have probably made other plans based on getting on that flight so I wouldnt normally expect too many changes to those numbers. For a MIA flight in March id expect a few more standby bookings next week when MF get their rosters and a few choose to take companions with them
Then there'll be lots more changes in the week before travel.
For busy flights, later changes can actually work in the favour of a single person on standby. Colleagues planning to take their entire family often bottle it and drop out.
Ive gone to bed the night before a flight showing 6 spare seats and then woke up to find it overbooked by 1 - but I still got on