I think it is reasonable, even likely, that the shield performed worse due to insufficient sample size. You can see the 95 CI bars are quite large. That's why I was very careful with my conclusions. My suspicion is that the 40AC is good for 1-2% mitigation which the test is simply not measuring. But if I were Kanras I would definitely be rechecking the code for a possible bug.
Sak was at like 170/210 item AC for this test which I think is a pretty reasonable range for most people I think (a full set of indicolite is 155AC according to the wiki). One unknown factor in this whole business is the Iksar AC bonus and how its screwing with things.
Oh, and one last thing. I wrote a little sim code. Even if AC works HP is still by far the way to go IMO. Here are the simulated results for Sakuragi with various amounts of HP against the Spiroc Lord with evasive disc up (I took the numbers from two parses). So at 4500 HP, in order to have a 90% of surviving against the Lord for 100 seconds, he would need a complete heal every 14.5 seconds. AC0 means 15 and 5% chances for max and min hits; AC1 means 14 and 6% chances for max and min hits; AC2 means 13 and 7% chances and so on. I hope all that makes sense.
So according to these sims, even reducing the max hit chance from 15% to 5% (AC 10), which would probably require several hundred item AC at least, is only worth about 600 raw HP. Those results make a lot of sense to me, because even at AC10 you are basically flipping one hit from max to min. If you are getting a CH every 20-25 hits, its quite possible that you could just get unlucky once or twice, whereas HP provide more space for the hits to return to the norm. You can in fact see that each HP is worth more than the previous one. (4500->5000 is worth an additional 2 seconds per CH, but 0->4500 is worth only 11 seconds not 18).
Of course that analysis assumes AC is working . . .
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caveat lector: i may have had a bug in my code
Increasing HP (4500): 90%: 14.5, 99%: 11.5, 99.9%: 10.5
Increasing HP (4600): 90%: 15.0, 99%: 12.0, 99.9%: 11.5
Increasing HP (4700): 90%: 15.0, 99%: 12.0, 99.9%: 12.0
Increasing HP (4800): 90%: 15.5, 99%: 13.0, 99.9%: 11.5
Increasing HP (4900): 90%: 16.0, 99%: 13.0, 99.9%: 11.5
Increasing HP (5000): 90%: 16.5, 99%: 13.5, 99.9%: 12.0
Increasing HP (5100): 90%: 17.5, 99%: 14.0, 99.9%: 12.0
Increasing HP (5200): 90%: 17.5, 99%: 15.0, 99.9%: 12.5
Increasing HP (5300): 90%: 18.0, 99%: 15.0, 99.9%: 12.5
Increasing HP (5400): 90%: 18.5, 99%: 15.0, 99.9%: 14.0
Increasing HP (5500): 90%: 19.0, 99%: 15.5, 99.9%: 14.5
Increasing HP (5600): 90%: 19.0, 99%: 16.5, 99.9%: 14.5
Increasing HP (5700): 90%: 19.5, 99%: 17.0, 99.9%: 14.5
Increasing HP (5800): 90%: 20.5, 99%: 17.0, 99.9%: 15.0
Increasing HP (5900): 90%: 21.0, 99%: 17.5, 99.9%: 15.0
Increasing HP (6000): 90%: 21.5, 99%: 17.5, 99.9%: 15.0
Increasing AC (0): 90%: 14.5, 99%: 12.0, 99.9%: 11.0
Increasing AC (1): 90%: 14.5, 99%: 12.0, 99.9%: 10.5
Increasing AC (2): 90%: 15.0, 99%: 12.0, 99.9%: 10.0
Increasing AC (3): 90%: 15.0, 99%: 12.0, 99.9%: 10.0
Increasing AC (4): 90%: 15.0, 99%: 12.5, 99.9%: 10.0
Increasing AC (5): 90%: 15.0, 99%: 12.0, 99.9%: 10.0
Increasing AC (6): 90%: 15.5, 99%: 13.0, 99.9%: 12.0
Increasing AC (7): 90%: 15.5, 99%: 13.0, 99.9%: 11.5
Increasing AC (8): 90%: 16.0, 99%: 13.5, 99.9%: 11.5
Increasing AC (9): 90%: 16.5, 99%: 13.5, 99.9%: 12.0
Increasing AC (10): 90%: 17.0, 99%: 14.0, 99.9%: 11.5