Same material or not, womens clothing is often more labor intensive to make and therefore more expensive than mens clothes.

It's a shame they didn't delve into why the rates were set as they are, it'd be nice if the International Trade Commission had to document a justification for their rates, but the article does mention that women's underwear is sold at higher volumes than men's, and are likely more expensive to start with too. The idea that we've been increasing taxes on items which are both more expensive and sell more often doesn't really surprise me.

Sales volume may also help account for why more high quality clothes made from expensive materials that are purchased less often get lower rates than frequently purchased low quality clothing intended for the working class (the majority of purchasers).