I was confused about the three required total scores too since the WIAT HAS 2 composites in math. It is my understanding that math composite (numerical operations and math problem solving) is sufficient for the total math required for DYS.
http://giftedissues.davidsongifted.org/BB/ubbthreads.php/topics/191061/WIAT_question.html#Post191061

I can't answer your questions regarding the discrepancy regarding scores, but have you seen the article outlining predictions for WISC IV and WIAT III? HTH.