I mean, I'm aware of growth spurts, but it's not guaranteed at all xD, all a roll of a dice even if you got tall-ass parents. There are plenty of dudes who are stunted even if they got parents who are tall af, and same goes with the opposite. Even if you take supplements, drink a lot of milk, excercice and sleep properly, etc...it's still not guaranteed, it'll help you fs, but doesn't mean you'll be guaranteed to become Lebron James. Not everyone's that lucky xD.
Growth spurt is one thing, I just think it's weird that bro just simply never had a single shred of doubt and that it's like he fully expected to get that tall? Maybe the manga just didn't explore into that, but I suppose it's not fully related to the story so whatever. It feels like a bit of a cop-out tho.