Then that would seem to argue that it's not so much the force of the bullet but the shock of getting hit.
And the more force applied the greater the shock.
The less shock absorbed by a vest the greater the shock.
The less ready the person being shot the less able they are to maintain their balance.
You can knock a man over with a light push from the palm of your hand if he isn't expecting it, if he is expecting it you might not take him down with a tackle.
When you are standing on one foot awaiting a shock that you know is coming almost every muscle in your body is involved in micro movements to maintain your position, the brain and spine are fully attuned to keeping you upright with no other function on the menu.
When walking along unaware of an immediate threat its orders of magnitude easier to get knocked down.
I went over a cliff face once because a tiny red fox kit jumped out from under a bush a brushed my ankle as I took a step while walking along a steep game trail.
Very nearly bought the farm that day.
Its not like school boys tipping over a cow.