20W charging limit through dock

Charging my OLED deck for the first time, I noticed the charging seemed slower than expected - reviews said it charged faster than the LCD.

A bit of experimenting and looking at the Energy display in the desktop showed a pattern - at mid-charge levels the OLED deck can charge directly from a power bank or the PSU at around 30W. But when being fed through the dock, it seems to not go above 20W, equal to the maximum rate of the LCD version, meaning overall longer charge time.

Superficially, this might seem to make sense - maybe the dock might be only designed to supply sufficient charging power for the LCD.

But then it doesn't really, because all the dock has to do is supply 45W in total - it can't tell what the power's being used for, and the power supply is still 45W. So where's the 20W charge limit coming from?

I guess it has to be a limitation in the deck itself? Is it reserving more power for real usage rather than charging, because it's attached to the dock? Even though not currently doing anything taxing?

Anyone else seeing this?