Merely speculation...
Some chargers let you choose options.
Example, battery voltage 6 or 12 (or 24).
Example, charge rate maximum or regulated.
I have a car battery charger rated 10 Amps. On the front is a switch choosing between two modes, automatic or manual.
Manual delivers maximum current indefinitely. The Ampere gauge goes to 10A and stays there. It's a fast rate capable of overcharging a battery.
Automatic mode reduces charge rate gradually as battery voltage rises.
The Ampere needle starts at a high value, jittering continually as charging drops to a trickle.
Perhaps your system also has options?