Looking at this again. I wonder if it needs to maintain peek charge rates. After all once the battery is full, it is full. Until something drains it down there is no need to charge. So to me it seems like the charger outputs full rate until the battery is full, and then steps down to a lower rate. They when the battery charge drops enough the charger steps up to the higher rate. Repeated over and over.
The reason the lower output chargers continuously output at max rate is because your load is high enough to prevent the battery from getting full.
My EVs do the same thing with regen braking. If there is not uncharged space in the battery (i.e. State of charge is high), no regen braking since there is no place for the charge to go.
Try this. Run what every load you are running. Unplug the charger. Let it run the battery down for a bit. Then plug in the charger. If it shoots back up to high charge rate good. And if it starts doing this cycling after the battery is full the mystery is solved.