Help me make sense of this sizing

[Mumteedum]

I'm looking for a hoodie for my nephew. He's a lanky 16 year old so I'd probably go small or medium men's. I'd guess that's usually a 36-37 inch chest?

These seller has this in a chart and I don't get it. I asked them and they just sent it again and said they recommend sizing up.

Maybe its just across the front of the body so you double the measurement for the true size? E.g. the medium says 21.5 so double that would be 43 inches (just an idea anyway)

Looks like the chest measurements are armpit to armpit in inches so the small would be 2x 20.5 so 41 inches so would give a few inches ease for a 36 in chest

Ah thanks.. I did wonder but then it still didn't quite seem right. Yes, probably would need a bit more room for a hoodie than a t shirt. Thanks