It is a level 45 mob. At level 45 it wouldn't be unrealistic to expetc a ranger to be in banded or maybe Ivy Etched in era - maybe 100 worn AC. You are a level 60 with 250+ worn AC.
To be critical, a potential flaw in the methodology could be your level/ac/defence are so high compared to the mob's attack that the attack is completely squelched, so adding more AC doesn't really do anything.
I suggest keep stripping off AC until you find the point where mitigation starts to noticably worsen.
Once you've found this point, you could retry the methodology with a mob of a presumably different attack rating. By doing this you could compare whether the observed cut off in mitigation/ac returns is due to excess worn AC squelching the mobs attack (which should be at a different point for each mob depending on it's attack), or there is a universal hardcap with 0% softcap return for rangers (there should be a similar mitigation for all mobs over a certain attack value at the same worn AC value).
Alternatively you could use other classes to demonstrate mitigation improvements with more worn AC against the Storm Giant Escort (is she expensive?), if you could do that it would be good evidence for the ranger being broken.
Moving past criticality, I do think you have demonstrated that ranger AC does need to be investigated. I have recently posted a bug thread raising concerns, linking to previous threads. It may be worth going to that thread and linking to this discussion to provide your futher (excellent) evidence.
https://www.project1999.com/forums/s...d.php?t=435250