I would have thought that ATI would be 'cherry-picking' the R420 cores which could run at the lowest voltage. This is what AMD did with their 'mobile' AthlonXPs, I believe which is why they tend to be good overclockers in conventional systems. In the case of these Mobile Athlons, they should actually be considered better chips than the standard ones. If this was the case it wouldn't necessarily indicate that they were struggling to find cores which are 16-pipe capable.
All speculation, of course.
All speculation, of course.