It’s not that you had to match the widths, a defined width was just the easiest way to trip “haslayout” for IE6 in this case. 
As Ray said all it needed was haslayout. zoom:1; woulda also worked however it isn’t valid. THe width was the easiest to set because it’s valid and safe :).
You can find a list of all the haslayout properties/values in the list Ray gave you. It depends which to use in a certain situation though 