
Click cell C4.
Note the formula in the Formula Bar:
='Numbers PerFrog'!F8*Sales!B4
This means Excel is multiplying cell F8 in the Numbers PerFrog sheet with January's frog sales.
When the formula in B4 was moved over to cell C4, Excel moved over the cells used in the formula, too:
From E8 to F8 in the Numbers PerFrog sheet...
...and from A4 to B4 in the Sales sheet.