Weird effect stacking

Prot spirit with shielding hands:
Reduces shielding hands damage first, then prot spirits reduced damage down to 10%.

Shelter with Union:
Reduced damage down to 10% with shelter, then reduces 15 damage with union.

Why are the same abilities reversed on different skills?

I was just curious

It depends on the order in which they are cast. Oldest triggers first.

Actually, I am pretty sure that Prot Spirit always triggers last. Therefore, you can't use anything to reduce the damage further. If you have both Spirit Bond and Prot Spirit on you, regardless of order, Spirit Bond triggers if damage = 81 or more.

well those target a signal ally, spirits work for everything, so its fair as i take it.

I believe once I soloed Hydras with a Mantra of Flame and it came after prot spirit. Maybe because it's a stance? I don't know.

No. It doesn't.

I know other effects stack the way they do, I just noticed this the other day and it seemed strange to me that 2 pairs of skills with the exact same effects would stack differently. I was wondering if arenanet made it that way for a reason, by mistake, or they flipped a coin on it or whatever.

umm...its been my understanding that anything you are under the effect of stacks in the order it was applied.

how did you test this? feel free to show some data and prove me wrong.