Remember - a base jump starts at 0mph, you go (mostly) straight down A powered exit imparts the plane's speed to a jumper, you fall in an arc gaining speed in the vertical, while bleeding speed horizontal.

This affects pilot chutes considerably. I know people who have done balloon jumps who remarked about how long it took to get enough speed for the pilot to be effective.

Keep in mind that an opening parachute sees the relative wind, which is zero at the start of a base jump.