It seems like the issue may be with the borders affecting your calculations. The top row of cells may not have borders, causing a discrepancy when setting widths such as 1200px. In CSS, the border needs to be factored into the width calculations for each cell, which can lead to offsets. Additionally, if there are cells without borders in the second table, this can further complicate the layout.
In the first table, the top row may end up being slightly larger than intended due to the border width added to each cell's width. The second table might have the opposite effect, with the top row appearing smaller because it cannot accommodate the borders properly. Merged cells in the tables could also contribute to these issues.
To troubleshoot, you could temporarily remove the borders to see if that resolves the problem.
If the borders are indeed causing the discrepancy, using jQuery's outerWidth() method to set dimensions instead of relying solely on CSS might provide a more accurate result. This method includes padding and borders in the calculation, ensuring the correct width is applied.
For more information on outerWidth(), refer to http://api.jquery.com/outerwidth/#outerWidth-value