As you raise a specific point of land, you'll quickly see that it forms a square top, always with two sides running perfectly north-south and the other two running perfectly east-west, giving you somewhat of a view of the world grid. The grid resolution is 1m between points and represent the pixels of a heightmap image, with the corners of every terrain modification being a specific point on that image. Because of this, most of your earthen wall segments will only come clean when running north-south or east-west, with other directions being a jagged collection of boxes, like building from Legos (the terrain system will sometimes clean up angles on the interior side of a corner, but not reliably).

This is also why you can't dig caves into regular dirt and instead just collapse everything from your pickaxe impact point up. It's also why every terrain modification will seem to have a 1-meter wide "buffer" slope around it.

If clean corners, good alignment, and accurate spacing are of high importance to you (and you're not opposed to using a QoL mod), then ValheimPlus[valheim.thunderstore.io] has a GridAlignment toggleable option that affects the operation of both the Hammer and the Hoe to keep everything matched up to the world grid when active. Keep in mind that it can be really finicky with the Hammer as it also constrains Hammer construction vertically - not just on the surface - so I suggest only using it for putting down reference objects (such as posts, beams, or small floorboards), toggling the option off, and then extending your main construction work from those.