Question about Anti-Freeze flush on NON-FWC engine

[DEcosse]

I bought a Reservoir with change-over valve and 5 gals of Pink -100 Anti-Freeze to Winterize my motor.
Today I changed the oil and filter, but have not yet done the Anti-Freeze as I have some other things to do before that last step.

The process is pretty straight forward, but I'm a bit confused about one aspect - after shutting off my engine and removing the hose and re-fitting the plug (flush hose goes in at the top end) I noticed some water still leaking out of the outdrive.
Which got me wondering - why do you not lose ALL the coolant you just filled? It is purely because with the hose adapter capped once again, there is no way for air to get in either through there, or past the impeller and so retains (most of) the coolant captive, other than just a little?