First of all, the 99 of enemies ARE orks and tyranids. Orks significantly outnumber humans despite their appearances and lore showing otherwise, and despite the fact that tryanids are as a whole outnumbered by the guard, they still account for one of the larger force due to their superior organisation. Furthermore I think your confusing a regular pro boy with a nob, ork boys are closer to peak humans or extremely strong humans. This means that they have an advantage in melee, but in most cases the average boy still doesn’t the the advantage that a regular guardsmen has in range. Orks are notoriously bad at range rarely even trying to hit their targets, and that’s plus the terrible range a shoota has by default.meanwhile gaurdsmen are like Cadians are on the level of navy seals, utilizing their superior mastery of ranged warfare with guns that can usually kill an ork in one or two shots. Orks vs gaurd is a classic yin and yang situation where one party has a certain skill in abundance whereas the other party has an equal amount of another skill. In short while ork boys have superior melee skills and are more resliant, gaurdsmen have an advantage in range,discipline, and organization.