Welcome to Coffee Snobs.
I wouldnt be calling 20-21 fine as opposed to 23-25.
Much of a muchness to me.
Fine is around 13-15 (espresso setting).
What brand plunger?
What size plunger?
How much coffee are you using?
How long do you brew the coffee before plunging?
How hot is the water you use?
The bark looking stuff is probably chaff; not a worry.