Cells have padding and spacing by default, so will not always add up to what you think, but this may have nothing to do with it. If you make your browser window narrower, you may be able to see what the client sees.
Now, though FP is notorious for not being the best editor, in this case I'd save a backup of the current page and see if I could get a working copy to look right in FP, because FP is obviously accounting for something that you are not. But this may still not fix it.
Other than that, we would need to see the actual page. It looks and sounds like you have set the position of the table, rather than allowing it to center, but I cannot tell for sure without seeing the page.
There are many other browsers around than the ones you mentioned. Are these the only ones you are concerned with?
Please post a link to the page on your site that contains the problematic code so we can check it out.
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