During their melee attacks / charges on the player, infected often duck down, and when they strike the player sometimes the players attack on the infected is either cancelled or somehow hit off target, this seems to have become a lot more frequent over the last few patches but I don't have any evidence or testing to back this up, only personal observation.
With the removal of attacks stunning infected with 1.12, this effect of them cancelling the players swings and dodging the players swings as they duck is more noticeable and quite annoying when trying to fight them with melee, please see the video below:
When swinging weapons at or around players, they seem to have a slight 'lock on' which allows them to land hits when they are slightly below or to the side of the swing which infected do not, perhaps this effect could be applied to infected to make hitting them a bit more reliable.