Jumping from aeroplane + physics + velocity

If I was to jump from an aeroplane and expect to land in a certain position but end up landing a few km away instead, how would I explain the reason for this occurring in relation to physics and velocity?

It depends entirely on what you expected in the first place.

If you had expected to drop straight down this couldn't happen because when you left the plane you had velocity in the direction of the movement of the plane.

If you expected to continue at the same speed as the plane this couldn't happen because there is air resistance and your horizontal component would gradually reduce.

If you had taken both of these into account and STILL got it wrong then you changed your shape during the descent to either increase or reduce the air resistance.

And of course if you deployed a parachute at different heights you would get a different answer even if everything else stayed the same.

But the simplest way to explain it all is " I never was much good at physics so it is my teacher's fault"
Or "somebody got this bloody map wrong again"

You have to include parameters like inertia, airspeed and your body position.
If your aeroplane is faster than it was supposed to, you will land away because of inertia.
If airspeed is higher, you may be a little blown away from your position, and
if you spread or fold or straighten your limbs, it changes your position because of aerodynamics.

You wouldn't...you never said the parachute deployed....