I was not considering the press point. I was using the center of each key.

In a column staggered layout I’m calling the distance between the centers of adjacent keys in the same column the row spacing. The column spacing is the distance of imaginary lines drawn along adjacent columns (through the key centres). I measure the shortest possible distance, which is at a right angle to those lines.

In a row staggered point of view it’s the other way around.

The different spacing comes from the hexagonal key shape. If you think of it as row staggered, the keys have 21.5mm horizontal (column) spacing and 18.6mm vertical (row) spacing. Rotate your point of view by 30° and this flips to a column staggered layout. Now the columns are spaced 18.6mm and rows are 21.5mm apart.

Square keys don’t have the same hexagonal symmetry. When you look at it as row staggered, it’s normal MX spacing, 19x19mm. When you look at it as column staggered, you need to do some trigonometry. The column angle is atan(0.5) = 26.6°. the column spacing is 19mm × cos(26.6°). The row spacing is from Pythagoras sqrt(19^2 + (19/2)^2).