The cb in all those stand for Comic Book(I assume) while all the end ritters stand for the actual type or archive it is.
CDisplay was designed with comic books at first.
cbz = Comic Book zip file
cbr = Comic Book rar file
cbt = Comic Book tar file
cba = Comic Book ace file
Naming a rar file into a zip file is kind of wrong, but with most programs these days, changing the extension rarely changes how it works in its intended program.