"If an IP address is statically assigned on the device (=printer for instance) this IP address will not get excluded from that pool and may clash with it! That device doesn't use DHCP and has a "real" static IP."
I am aware of that, that's why I have addresses .002 - .100 free because occasionally I need those for testing purposes with network devices that required static addressing to work properly. (Domain controller with dynamic addressing = The road to ruin.)
While this manual assignment doesn't require free address space it's anyway good idea to keep some range of addresses outside lease pool.
As you can see my example of my router setting, Buffalo use "Manually Assignment".