Spells: Druid swarm line

[Taluvill]

Had an interesting encounter with the druid swarm line today. This is on the PVP server, RED.

Druid A casts creeping crud, it sticks.
Druid B casts creeping crud, it does not stick.

Druid A casts Creeping crud, it sticks.
Druid B casts stinging swarm, it sticks.

Seems like when different druids use different spells of the same line, they stack, but when they use the same spell in the same line, they don't stack.

[Szeth]

That was how I understood it worked? Can you show some sort of linkage to a different mechanic?