BEAGLE:
At the planned (opening) altitude the parachutists would have been at a certain velocity, but not at terminal velocity.
Then I am more confused. It takes about 15 seconds for a human body to reach terminal velocity. Less if they are encumbered with heavy equipment like weapons. I've never encountered a military freefall jump where the freefall was planned for less than 15 seconds. In my experience, such a short freefall would be pointless and a static line jump would be preferable. And if it was planned for less than 15 seconds, they'd use a stop watch, not an altimeter to determine their opening point. Brits (or whoever did this jump) must have very different tactics or procedures than the American tactics/procedures I'm familiar with. That explains things. Thanks for the clarification.
Did anyone else not understand that line of thought from my original post?
Pasta clearly did not. He offered an entirely different explanation.