Do legal departments pay law firms more quickly than their companies pay other vendors?

I ran into a study that found the timing of payment to law firms varied significantly around the company and that the particular company often paid outside counsel more rapidly than it paid other vendors. Unless prompt payment discounts apply for law firms, this should not happen (See my post of May 11, 2008: prompt payment with 10 references.).

Much as I respect law firms, they are vendors in the eyes of corporations and ought to be subject to the same fundamentals of vendor management and payment as all other third parties who serve a company.