Arcane Momentum only procs on direct cast (description implies otherwise)

[KFears]

Arcane Momentum passive in Sorcerer’s tree says (original grammar preserved):
When you cast a spell you gain a stack of Arcane Momentum for 2 seconds and refreshing the duration of all existing stacks.
This description implies that casting a spell is a trigger, but the actual trigger is directly casting a spell. I’m playing a Spellblade that casts Lightning Blasts on Surge, and Lightning Blast skill tree explicitly states what applies to direct cast, and what applies to cast (e.g., Lightning Attunement node applies to any cast, and Arcing Power applies only to direct cast).
I’m not sure if this is a bug or an inconsistent description, but I was expecting the passive to work on any cast, so when I found out I had to respec a little.