But only one side functions as a dampener? That is hard for me to believe!
Dampening is what you do to a washcloth. :giggles
If you want to sort this, Bizz, it'll be easy if you emptied both fork legs thoroughly and filled them both with 400ml of fluid. Drain the fluid from the fork legs using the drain holes in the sides of the fixed tubes. The side that has the least fluid is the side without the drain holes in the cartridge. After that, you can refill with the correct replacement amount.
In their current state, your ride won't be affected but if you want to replace oil without total disassembly or flipping the bike wheels-up with a chainfall assembly (post photos if you do), you'll know which leg gets less oil and won't be overfilling.
BMW workshop manuals are created for factory-trained technicians. Their diagrams tend to be more specific than Clymer or Haynes. Their text is less specific because factory training fills the spaces. Clymer and Haynes tend to have more descriptive text because their readers benefit from that. I use all of them when engaged in an unfamiliar procedure.
Here is the diagram from the BMW manual. The Clymer manual doesn't have this diagram although it has used many of BMW's others. It shows how one cartridge in the K1100 fork assembly differs from the other.
