Why are some row numbers skipped on some planes?

In the past few years many airlines have standardized their row numbers across all aircraft. For example, rows 1-9 may always be first class, rows 10-20 always premium economy, and row 21 may always be an exit row. Let’s say a particular aircraft is configured to have just 5 rows of first class and 8 rows of premium economy. In this case first class will have rows 1 through 5 and premium economy will have rows 10-18. Rows 6-9 and 19-20 will be missing. This was done to allow frequent flyers to better understand and compare seats on any plane, and to allow the airline “room” in the form of available row numbers to add to the first or premium economy sections.