It just might be his age. In the 90s "X" was super cool in the culture that people his age may have been into. In particular comic books. X-Men, X-Factor, X-Force, Gen-X, etc... No clue though.
But he was a much more “on-paper” billionaire because his wealth was tied to some oil companies that had that valuation only on paper. It was tied to finding a lot of oil on some places. When that didn’t happen, all that “wealth” went to dust and he was back to being “only” a multimillionaire.
chop