In that example they're considering the single variable to be a sum of a lot of different factors. Like how you might consider the amount of time it takes to drive to work to be the amount of time it takes to drive from home to point A, from point A to point B, from point B to point C, and from point C to work.

The amount of time it takes me to write this reply can be thought of as the sum of the amount of time it takes to write each word. Sometimes people like to think about their data as derived from other variables in this fashion.