the reason it’s not obvious is because a lot non projectile procs can still count as projectile
most obvious offender is ofc Volcanic stride, “ground effect” = projectile for the purpose of triggering the aura, buuut not for the purpose of using deflect 
when i was experimenting i think it turned out some debuffs like Assassin’s mark or matriach ring? counted too (i don’t remember which), and that absolutely floored me even more than the Volcanic stride revelation
Double “fun” fact, because those very same things that do count as projectiles for triggering the retal aura does then not count as “projectile” for triggering shield block 
that was a, less fun, recent experience learned, - “rules for thee not for me” 
(shield block v projectiles is a whole other can of weird worms/inconsistencies)