right,
seen your replay

the knights started as valour zero,
i am uncertain if valour gained in battle has any effect on the units combat ability or not,

next, there was a fatigue difference between your knights (very tired) and the handgunners (quite tired),
next you lost a man to fire from the handgunners in charge, that caused a morale drop which may have had an impact of fighting ability,
the charge cost you both one man each - the HG looked to be in hold formation, which may be the cause for the low casualties from the charge, plus your knights were very tired and may not have reached charge speed, and thus may not have got the bouns,

your knights spent several "rounds" afraid of guns, thus they had low morale for those rounds,
next you lost half a dozen simultaneoulsly again causing a morale problem,

the "brawl" went man for man for a while, but then 5 gunners clumped together and cut down every knight who came at them,

the gunners may also have had a weapon upgrade or two,
the owner was quite willing to commit them h2h even in the early stages of the battle, makes me think they were a bit beefier than you can see from the replay.
BUT (one nice thing)
when your knights routed they ran down three gunners as they fled and lost none themselves in fleeing...

i think fatigue, afraid of guns and maybe a weapon upgrade or two for the gunners made the difference,
also the clump of them off to one side, being attacked by one knight at a time and cutting them all down would not have helped at all...