As has been pointed out many times, there was / is no logic to Jaguar model
or powerplant designation. In the old days Bill Lyons named them all, then
it became the province of the marketing types who felt they had to protect
the "heritage" aspect of the brand and wanted continuity. Thus we now see
that the big sedan is the "XJ", the 2 seater is the ressurected "XK".

When the mid-range sedan was being developed many of us working for the
company at the time wanted it to be called the "Mark II", but there was a
lot of concern that the term "Mark" would be associated too strongly with
the Ford ownership & Lincoln's Mark series. Thus the old "S-Type" name was
resurrected. As for the "X-Type", it was strictly a marketing decision, as
Jaguar never had had a model in that segment before during modern times.