It’s the difference between lead time and production period.

Let’s say GM rolls off 1 car out of the assembly line every hour, it doesn’t mean the work they do to build a car only takes an hour.

Maybe it takes a week? (I’m guessing) to get from frame to finished vehicle. It means the first six days are just filling the plant line, and 7th day they plop out a car an hour as they add one frame to the input at one an hour, but lead time is a full week to get car X from start to finish.