The real difference is phases vs turns. AFAIK in phases casualties are added up and based on the overall amount of casualties the check is made. so not matter how few or many units fire on an enemy unit, if they would amount to 25 kills you would have that 50:50 chance of triggering a moral check.
But that obviously works differently in turns were you can get disorder right after firing on a unit. In turns these checks are based only on the result of the just occurred casualties and not on all casualties that the unit received in that turn. So firing on a unit gives instantly the result of disorder or not. This is afaik why there is a OR in the Panzer Campaigns series that delays the report of disorder, so you are not sure if a unit just got disorder by your fire or not.
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Lieutenant General Christian Hecht
Commander I Corps, Army of the Potomac
"Where to stop? I don't know. At Hell, I expect."