Bard.. squishy

[Diogene]

I got one lvl21bard and one 19 cleric. Both share roughly the same HP’s and AC, and also defense skill. Both are logged in permafrost. Bard takes high damage hits while dmg mitigation on cleric is so much better. I just dont get it. What am I missing here ? Heck, bard got 600AC and cleric 580 and he’s two lvl’s under. I played both long enough to witness. Is there any special rule for bards being so squishy ? Btw bard agi is 100 and cleric 90.