Unique net boot CD required for every version of SuSE?

I have several versions of SuSE as installation sources on a server. Sometimes I install from them via PXE boot (whenever the PC in question is setup for that) and sometimes I use a net boot CD to point to the installation repository on the server.

When using a net boot CD it has to match the very exact version of the desired SuSE version to install. Why is that?

I mean, the installation system is loaded from the server anyway so why is the net boot CD made to require a certain version of SuSE, and certain architechture too for that matter?