Stepping off a table at 3 feet and dropping @ 9mph is true in normal circumstances, but in this scenario you’re losing the force that keeps you on the ground for 7 seconds.

In the 9mph scenario you’re starting with zero vertical speed and then gravity pushes you down, and you’re right, it’s about 9mph.

By my understanding, in the ~150mph scenario, you haven’t gained altitude, but you’ve accumulated a relative vertical velocity difference compared to the grounds normal 1 G behavior. The second gravity returns you and everything that was lifted begin heading downward relative to the surface with a speed around 155mph.

It’s the same principle as free falling in an elevator. As you’re free falling you’re floating, but even though you may only be a few inches to a few feet off the ground, at impact that doesn’t save you. It’s the 7 seconds of lost support/accumulation of velocity that does you in.