It has to do with when they are cutting the chuck off of the hanging carcass, the stroke of the knife, if you are really interested in maximizing the chuck, drags across the brisket in that spot where all briskets have that slice. The chuck is cut to be maximized, especially now that the chuck tender and under-blade cuts are selling at a premium.
It doesn't have to be, but, briskets are still an afterthought to the market.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"perhaps...but then again...maybe not..."