1) marketing

2) Some "backpacking boots" are stiffer and have things like a full shank, or 3/4 shank - making a stiffer boot than a "hiking
boot" - but that's a general statement, not a rule per say.

knowing if it's a full shank, 3/4 shank or half shank boot
is probably more important than what label someone gave it to
try to sell it.

But the proof is on your feet. not in what it's called. if it
fits and you like it, wear it. if it feels stiff and nasty, don't.