The existence of such a variable is a common misconception caused by the relatively high spread of early game weapons causing shots to often land quite a bit away from the center of the reticle. As we can see, the player is perfectly accurate on every difficulty in every game in the series, there is no hidden accuracy value and every bullet that hits an enemy will deal damage and is never randomly ignored. These values have been directly lifted from the game's data files.
Here's a table that shows the actual differences that the difficulties have ( courtesy of /u/Voroseeg from Reddit):
Please be aware that the other answer to this question is based on a common misconception of how the difficulty works, and is mostly false.
