Monthly sales jump around. A moving average smooths them: each point becomes the average of itself and the points before it. Quick Table Calculation > Moving Average averages the current month with the two before it.
In a calculated field you control the window: WINDOW_AVG(SUM([Sales]), -2, 0) averages from two marks back (-2) to the current one (0).
LOOKUP(SUM([Sales]), -1) returns the value one mark back: last month's sales. Subtract it for the change, or divide by it for the percent change.
The first marks have no earlier marks to look at. A moving average there uses fewer months, and LOOKUP returns NULL, so the first month has no change. That's correct: there's nothing to compare it with.