Could be several reasons why the charger is set on 21A but the monitor
only shows 18.5.
First, depending on how the monitor and batteries are wired, some of the output from the charger will be feeding lights, pumps or any other loads switched on inside the boat. So if the charger is really putting out 21A, then you could be putting 18.5 in the batteries and running the lights with 2.5A.
Another possibility, no matter what the capacity of the charger, if your batteries are charged then the charger will not be putting very many amps into the battery.
And, as you say, the meter on the charger could be approximate.