Change the code in E1 to =SUM($A$1:$A$3) . You can change other cells and it won't change the range.
you can do this, but in a spreadsheet you would probably have wanted to copy the formula.
Using the "everything up to current row/column" is a reasonable default for SUM, you can always change it.
But formulas changing because of the alteration of cells not in the range covered by the formula is not on.
Definitely strange it does that. Another way to do the formula is below where it offsets by 2 cells. You can copy the formula using this.