This here determines the way the table is calculated, regarding the width of the table and the width of the columns.

If you have set a table width and the combined widths of the columns are wider than that, because of content in the table-cells pushing the columns more apart, the table will be larger than what you said, if the table-layout is set to 'auto', which is the normal setting.

If you want the table to really obey to the width settings you put on the columns and the table itself, you have to set table-layout to fixed.
Setting the table-layout to fixed only works if you have set a width for the table!