lucy24 - 1:26 am on Sep 21, 2012 (gmt 0)
Are you certain about what you are seeing? On closer inspection I'm not so sure. Recalling outline is drawn around the edge of the border, so any variations are likely related to the interpretation of border-collapse (and for internal cells, maybe also stack order).
That's why I made a 3x3 table-- so I could see the behavior on cells in different locations. And, ahem, it's a table because that's what the OP was asking about.
:: detour to check something else ::
Things get interesting when you double the width of all borders. Then it becomes a variation between seeing the inmost 1px of the cell border, and seeing the whole thing. But I added a proper 4.1 dtd at the same time, so that may have an effect.
The one thing I can be sure of is that adding outlines to table cells has no effect on cell size or spacing. But if the cells already have borders, it would be safest to change their color (only) on hover.