Because it's all an average.
Say it's all potatoes, you can peel 4 potatoes a minute on average. But, sometimes you get small smooth ones that take only 7 or 8 seconds, sometimes you get big knobbly ones with lots of eyes that take 30 seconds plus. So you reach into the bag and get another tater and start peeling, sometimes you get lucky and get 10 small smooth ones in a row and your "rate" looks great, but then you have a run of big knobbly ones and you slow down again.
Thats a pretty good analogy, I like that.