I am not sure if you are saying this or not but if you are saying it is not possible for and an arrow to stagger you on impact you are incorrect. I have been hit in the bare leg (no armor) by a "light weight" 60 pound bow with a BLUNTED arrow (round hard plastic arrow head used for hunting birds). It was a stupid accident that was mostly my fault.
From that experience I would bet you a years salary that if I did the same to you, you would stagger as well!
I am not what I would call an "expert" in physics but I am an expert in medieval combat.
See here:
World wide organization (SCA)
I have been in the Top 1% ranking as King twice and now hold the rank of Duke as well as Knight. (my credentials: http://wiki.caid-commons.org/index.php/Patrick_Levi_Darkwrath )
My book on the physics of Combat: http://www.spookyfx.com/book/tromp.html
(I made the Duke Patrick mods. )
Stagger and knock back effects are not
only caused by momentum (relatively slow medieval arrows) or kentic enegry (fast modern bullets).
Pain
Lose of use for a moment of important muscles or tendons.
Pressure points for nerves and physiological weak points.
Balance (the act of walking is a constant cycle of losing balance, a even a small disruption in the cycle can trip you to the ground.)
Well, I'm no expert at Archery per se, but I am an expert in physics (which defines 95% of any sports, weapons, you name it). An arrow would not stagger anyone any more than a Shotgun would throw you 50 feet backwards. You either get hit by the arrow and get severely wounded, or you feel nothing.