my 13" MBP almost never maintains its 60 watt peak consumption. Even when the battery is around 10-20 percent its only around 45-50 watts unless im charging and hitting the CPU and GPU at the same time.
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.
USB-C cables are 100 or 60W. If the cable is not 100W certified and chiped you cannot push more than 60 W (20V @ 3 A) max. With appropriate chipping and certification you can push 100 W (20A @ 5V) max. And within these limits the Power Supply and Device decide what is the best Amps and voltage. So USB-C Power Deliver is not some sort of dumb pair of wires and dumb power supplies. This negotiation goes on to determine the max amount of power than can be safely delivered.