I couldn't find an specific answer between CM replies that explain why thorn spitter is a tier 3, but to classify the items they take into account availability, not only of the item itself but the amount of similar items, with this said, Thorn spitter is in a market with multiple other crossbows that are equal or harder to obtain turning them into disposable (and that's why chain bolter is t4 while thorn is t3).
Legs haven't the same availability, the market of legs is either too easy to obtain (most of them being class t2) or way too hard (t4), in between ( t3) lies the few items that aren't so easy to obtain with most of them being vocation oriented except by dwarven legs that also can only be obtained once through a quest, so the next other item with no-voc requirement obtainable just once and with higher difficulty that their counterparts is the Demon Legs, the last item with this characteristic is the Green Demon Legs which helps to understand the lack of availability, there aren't other legs that could replace with ease the Demon Legs in availability while there's a huge market of shields and weapons to replace any other reward from Demon Oak quest.