I suspect this due to the stroke settings on the surrounding cells.
The cell to the left of the 20 cell has a white stroke applied to the bottom and right sides and that white stroke is covering up part of the black stroke on the 20 cell
The 39.858 cell has a white stroke applied to the two sides and that is extending the "scope" of the cell.
See below, using colors so you can see what's happening
I have found with tables, when using table styles and cell styles, I have to often "clear overrides not defined by style" after I've applied the table/cell styles. It seems to hold on to previous local formatting in a way that text doesn't. YMMV.
Thanks for getting back to me. Unfortunately I'm now I'm on holiday for about a week so when I return to this project I'll take a look. I'm sure I tried to clear overrides but there was still some stroke issues. Thanks again, I'll let you know how it goes!