I notice this kind of thing in tournaments i have gotten around their side or have come up behind them while they are already engaged.
And instead of taking normal damage they take very minor damage as though they had blocked it somehow despite having their back to me.
Or without raisng their shield or sword to block the attack and no the sword is not hitting the shield i am attacking on the unshielded side.
I would also add to this that when troops are packed shoulder to shoulder face to face with an enemy you are unable to attack at all weapons just collide of friendly troops. I understand why this happens and accept the reasoning however certain weapons are desingned for this sort of fighting like using a spear from the 2nd/3rd ranks.
It takes the fun away from assaulting a castle if you cant hit anything because troops are packed so tightly.
I have tried shorter weapons and still have the same issue I refuse to assault a castle with a sodding dagger though before anyone mentions it lol.
That is something different from the thread openers issue.
Your issue stems from enemies with high armor being hit with weak weapons and skills, which results in too little damage taken and the game counts that as a bounced off attack. Happens a lot in Arenas, where you have lords with the best possible armor, while having to use some the worst possible weapons against them.
The second one is a well known balancing problem regarding the collision of units.
Basically it's either the player being blocked by units too much or the units stacking up too much.
That issue need a lot of fine tuning to allow the player to pass through friendly units, but also stop units from stacking too tightly.