Dilution can also happen through a reverse-split prior to IPO to hit a target share price. (In this case, the bankers effectively dilute everybody else, although C-levels will probably get big new post-split grants to stay on after the IPO). The number of shares (rather than the percentage) might be more important given an IPO will probably require $10-15/share, and the share price will probably be no more than 2x-3x that after 6-month lockup.

IMO telling an employee you have x% of $n billion is very misleading, as dilution can be rather unpredictable. You're really showing the employee the /max/ upside in 2-7 years, which IMO is a very weak sales position.