There are two popular keyboard layouts: ANSI and ISO. Your keyboards (both the BW and Zitech) have the ISO layout.
ANSI is the most popular in the US, and the layout looks like this:

ISO is more common in Europe:

As you can see, certain keys are shaped differently (e.g. the Enter key, Shift key). Also, some keys are in different positions or have different legends. Unfortunately for you, keycap sets are generally made to cater for ANSI boards, so that is what alaricljs meant by it being hard to find a set that fits the ISO layout.