Are you wanting to see how a specific group of students has grown over time? The Student Growth Summary Report that has been generated for different academic years (fall '18 to fall '19) represents the same group of students that tested in both of the term pairs you selected in the growth comparison period. Read more about this with Student Growth Summary report: Which grade are students grouped by?
To compare grade growth over time that do not represent the same group of students, you can use the District Summary Report. For example, if you wanted to use the District Summary report to see how students in 2nd grade in fall 2018 improved by fall 2019, you would compare fall 2018 2nd grade scores to fall 2019 3rd grade scores.