Good morning.
Is that normal that KSysGuard show that I have 15,6 GB of RAM, when I have (obviously) 16?

Mine is showing the same thing: I have 16 GB and KsysGuard shows 15.6 GiB total.
Gnome's system monitor also shows 15.6 GiB total.
Checking the total memory according to /prod/meminfo, I see the 15.6 GiB number is accurate.

Of course I'd have expected that memory usually comes in powers of 2 and thus that having 16 GB of RAM would mean that the total physical memory should be 16 GiB exactly (unless there is some buffer amount reserved by the kernel or something which is reported separately). Perhaps that expectation is incorrect and that RAM is usually within some tolerance of the stated quantity?

The difference is in the units the memory is quoted in Gibibyte (GiB) vs the ram you are quoting in Gigabyte (GB). One is base 10 the other base 1024. See: https://en.wikipedia.org/wiki/Gibibyte