Yep it's not all about THC. It's about the basket of cannabinoids and terpenes. Also about the ratio of different cannabinoids in the bud.

Definition of hemp both in EU in US served mostly prohibition. THC rich plants were curbed and the public was convinced that somehow we have two different species, which we know is pure bullshit as both CBD and THC are synthesised along the same metabolic pathway from CBG.

We also know thanks to folks at Phylos, that hemp is closely related to indica, which commonly produces copious amounts of THC together with CBD + is myrcene based in term of terpene profile.

It's just different cultivar bred for different reasons.