This bug report describes the same issue as report 26253. Only that bug report
was filed against OOo 1.1 years ago so maybe thats why its not being fixed.

Problem: Table border change causes spacing to contents to change

To reproduce this issue:
1) create a table in OOo (for example 2x2)
2) Click in the table and select “Tableâ€->â€Table Propertiesâ€
3) Click on the tab “bordersâ€
4) Default the "Spacing to Content" (table properties dialog, tab borders) is
set to 0.10cm for left, right, top and bottom. Change this to (for example) 
left 0.50cm
right 0.00cm
top 0.00cm
bottom 0.00cm.
5) change the border from one of the cells, using the “borders†tool in the
“Table†tool bar.
6) As you can see, not only the border gets changed but also the spacing to the
content. It looks like the value for spacing to the left (0.5cm) is now also
applied to the right, top and bottom.
7) When we open the table properties dialog again (tab borders), the "Spacing to
Content" settings are now:
left 0.00cm
right 0.00cm
top 0.00cm
bottom 0.00cm
If we change this back to our requested setting (0.5cm, 0.0cm, 0.0cm 0.0cm) the
table looks OK.

So, it looks like the "border" tool does more than just changing the borders. It
changes the "Spacing to Content" as well. This is of course very annoying when
using the borders tool often.