I got Critted with 275% Fort - what is Fortification doing?

[Circon]

(Combat): Illusionary Sprite critically hit you for 173 points of pierce damage.

This was in Partycrashers on Epic Reaper 1 mode, a level 22 quest.

I knew epic monsters could bypass some amount of Fortification, so my sorcerer was running around with 275% Fortification listed on character sheet. +100% from a Heavy Fort item (blue augment), +100% from being in Air Elemental form, +50% from Brace for Impact in the US Epic Destiny, and I can't remember the source of the last 25% right now but it might be a past life or something.

Does the Illusionary Sprite have -180% fortification bypass, does Reaper mode randomly crit you regardless of fortification, or what is going on here?

It feels very disappointing to have stacked so much fortification and then get critted anyway. I thought I had a good safety margin, but it still wasn't enough.

[TPICKRELL]

(Combat): Illusionary Sprite critically hit you for 173 points of pierce damage.

This was in Partycrashers on Epic Reaper 1 mode, a level 22 quest.

I knew epic monsters could bypass some amount of Fortification, so my sorcerer was running around with 275% Fortification listed on character sheet. +100% from a Heavy Fort item (blue augment), +100% from being in Air Elemental form, +50% from Brace for Impact in the US Epic Destiny, and I can't remember the source of the last 25% right now but it might be a past life or something.

Does the Illusionary Sprite have -180% fortification bypass, does Reaper mode randomly crit you regardless of fortification, or what is going on here?

It feels very disappointing to have stacked so much fortification and then get critted anyway. I thought I had a good safety margin, but it still wasn't enough.

Beast Mark and Archeron Tier 3 champions can ignore fort completely.

[Arkat]

Beast Mark and Archeron Tier 3 champions can ignore fort completely.

I knew some monsters could ignore a certain amount of Fortification but I did not know there were some that could ignore it altogether. That seems unfair!

I tried to +1 you, Grubby, but I've got to spread some more rep around first before I can give any to you again.