Description reads 300ac, but it seems to only give me around 90. That a bug or is the ac dependent on class like the Gnoll illusion clicky?
The spell description doesn't match with the actual spell data. Spell data says it should add ~90 AC (which is what you're seeing). So that raises the question of whether or not the description is wrong or the spell data is wrong.
Same reason you only get approximately 440 AC from Surety Rk. II when the raw data adds 1469; eq math and overcap returns drops that 1469 to 440ish displayed. The base 12 on Grit II is 300 so eq mach and overcap returns drops that to the 90ish displayed.
Overcap returns do not effect -displayed- AC at all. Matter of fact, overcap returns means you are SHOWN more AC on your char sheet than you actually get from an item. EQ math, however, is always a solid answer for these issues. Because well... it's wonky.