In a way it kind of makes sense though, the Chosen are picked from the masses as special for their ability to maintain their cool or to wreak devastation, so its only natural that the less common 'halfies' would have a better chance of breaking the mould so to speak and escaping the mass of pure-bred ordinary Orcs. If anything, a full-bred Orc would be more special in the Chosen, because they've overcome their full heritage to be considered worthy of being marked out as an individual. It would be comparatively easier for someone who's half human or half elf to control their temper or master a weapon outside of the norm.

Also, I suspected the Giantess would probably try and abuse the quick little comparatively handsome Grolan, it's a good job he's agile.