There is a difference between (actual) width and preferred width.
Preferred width was introduced in Word 2000 and it allows you to express
preferences which may or may not be the deciding factor in determining actual
table properties.
When you specify a preferred width for a single cell in a column of a
different width, Word has to decide how to interpret the conflicting values
it has. In Word 97, the top row (and only the top row) would have been
resized to accommodate the explicit request for a certain width; there wasn't
really any other choice it could make. In Word 2000 (and later) Word has a
choice and it chooses as you have seen. You may not like its choice but it
has a set of guiding principles; if you want something different you must
provide a different set of (possibly still conflicting) values and you may
have to use a bit of trial and error because I am not aware of any
documentation on how Word makes its decisions.
You can, if you wish, still use the old (Word 97) dialog and get the old
behaviour. There are three commands: TableFormatCell, TableCellWidth, and
TableRowHeight and they are available via the Word Commands list under Tools
Macro > Macros (Alt+F8). Alternatively you can add them to your toolbars
via Tools > Customize < Commands (tab), Category "All Commands" - scroll down
the list on the right to find them.
Enjoy,
Tony