Damage is based on several factors, which cause it to deviate massively from the maximum value. Assuming the exact same powerstrike, there are several factors that will lead to a wild variation in damage. The first is speed bonus, if you are moving towards each other, the maximum damage of the bayonet will skyrocket and will almost certainly be a 1-hit kill. The second is how long the attack is held; up to a maximum of about 1.5 seconds, the damage on a held attack will increase (the potential to stun the opponent will also increase). The distance from player to player will also affect the damage taken, too close and the attack will do barely any damage (or may even thud, which is when the attack fails to meet the damage threshold to interupt the opponents attack). Similarly being too far away will cause the attack to do barely any damage. There is a sweet spot here that you must get used to. You can trick this 'sweet spot' mechanic by deliberately moving your camera such that it takes longer to hit the opponent. The attack type also appears to affect damage, while not confirmed by any testing, down stabs appear to do more damage than upstabs.
Finally hit placement and armour also affects attack damage. Armour doesn't really matter with the exception of increasing the extent of the close and distant thud ranges (especially on sabre briquets). However, attacks hitting the head do the most damage, legs, potentially the least (not sure of this, because generally leg armour is less than body armour) and body somewhere in the middle.
There are other things which affect damage, even indirectly, like athletics affecting speed bonus, or attack type. But considering the bayonet is of piercing type and armour is not in play, damage is a bit more predictable than warband, and way more predictable than crpg
