When you specify the widths of the columns in a table, we try to show you approximately the total width. These are the column widths which we tell Word to use.
However, Word tries to “help” and sometimes changes these widths. Sometimes that’s useful, where a narrow column has some long words in it. Then Word will expand the column. But all attempts to fix the column width precisely have failed, and I’ve now given up. Word is just too “helpful”.