Two conditions make the cells square
The canvas must be square and the row count must equal the column count. The preset satisfies both conditions, so each of its nine regions has equal width and height.
Changing to portrait paper while keeping three rows and three columns creates rectangular cells. If the task depends on squares, keep the paper option at Square rather than assuming that 3×3 alone guarantees the shape.
Preserve proportions in the destination
A 2400×2400 PNG can be resized to a smaller or larger square in another application. Proportional resizing keeps the cell shape, although the visible line thickness changes with the image size.
Avoid dragging only a side handle or fitting the template into a nonsquare box. For a printed exercise with a particular cell size, set the overall print dimensions and measure the result rather than relying on the browser preview.