The background (board) grid appears to be in units of 24. Half of 24 is 12 and a quarter is 6. Design systems, video standards and font measurements almost always use binary units for example 4, 8, 16 as units of scale.
Take for example the standard video dimension HD which is 1280 by 720. If the grid unit was 16 both the height and width of the video would align perfectly with the grid. With a grid of 24 the width of the video is 1280 / 24 = 53.33333333333 units and... oh dear now I can’t snap spacing.
Of course you could always let users control their grid size which would be perfect but Miro doesn’t appear to be that kind of product from what I’ve seen.
