Columns won't reduce to lowest default setting of 8 px

I am trying to get around an issue where Numbers is now suddenly "deciding" all the blank white column gaps that I've been previously using for months -- which were 8 px -- are not allowed anymore and now making the new gaps 10px. Nothing has changed in term of colour borders around the tables between groups of data in this table. Even if I reduce the colour borders to eg 0.25px from 2px I still cannot get the blank white gaps between two groups of coloured cell data to go down to 8px?

When I leave a setting at 8px, why when I come back to the document, is it changing it to 10px? (see red arrows)I have adjusted individual data cols px width but have never seen it change already prefixed 8px blank cols to 10px. Hard to make the top and bottoms tables look symmetrical...

Thanks