How indeed? I'm going to go old School here and explain in chart format:
Why is this Low HP Orc here when "logic" and statistical analysis shows it would probably not?
1. Orcs don’t get sick days. Feeling under the weather, nursing an infection, actually injured so badly going to the fight might be a bad idea….well Mr. Orc you better show up or you’ll be killed,butchered, and eaten for dinner. Well maybe it will not be that bad but you don’t get a single copper piece or bite of food if you don’t at least show up for the fight.
2. The Orc is a Runt. For all the same reasons as above the orc is going to show up because he’s not just inconvenienced by a temporary health issues but by his normal condition, it ain’t easy being an orc.
3. The War God Hates this Orc. The war god has it in for this orc, the orc might not even know it until some elf sticks an arrow in them or a dwarf knocks their head off with a battle axe.
4. The Orc is new at this. The orc just got here, it’s a whelp, or it just crawled out of the breeding pit, however little orcs get made, this one is a fresh/young one.
5. This Orc is a Slave. The other orcs are well aware of how weak this orc is, they don’t care, they even think it’s a little funny, such wimpy orcs will be driven before the horde as arrow and sword fodder to give the bigger and stronger orcs a chance to close and slay their foes.
6. Hasn’t Been Killed Yet. Sure this orc should be dead if it faced any real injury or risk in it’s life, guess what, that’s right now, during this encounter.
7. Not Really an Orc. This critter isn't really an orc it's some sub-race or creepy half-breed, orcs can make half-orcs with things besides humans.
8. Orcs don’t know how many Hit Points they have. Plainly and simply orcs and all other monsters and NPCs don’t know how many hit points they have as they are not controlled by a player who enjoys access to this information to make decisions about their individual actions. While convention says roll 1d8 if a creature has one Hit Die the DM is also free to assign whatever reasonable Hit Point Score they feel is appropriate for an encounter , the Orc has no clue they have only 1 or 2 hit points and will keep on doing things orcish until they get clubbed, poked, throttled,cleaved, crushed, set on fire...