The extra 2 pieces is to take care of the table captain, and volunteers.
The arguement for not throwing out the low and high score, is that a score is a score. And the portions turned in can be much different especially with ribs and chicken, and can be with pork an brisket also. Each piece of chicken is different to some extent, and the ribs likely from different slabs, and pork from more than one butt or shoulder, and brisket sometimes the same. It also virtually eliminates ties.
Frankly I know throwing out the low score has, and likely will continue to be discussed. Throwing out the high score will probably never happen.