In Mark II, both the Sentry and Shifting Stones had the old Immobile, which included Circular Vision in the entry for that rule.

However in Mark III, Immobile and Circular Vision were separated (and changed). Shifting Stones retained both Immobile and the new Circular Vision... but the Sentry Stone is conspicuously missing Circular Vision.

My question is: why does Sentry Stone not have Circular Vision? I just got back to the game and I thought this might have been an oversight, but now with the first errata released, it seems that this is now a deliberate choice. I know how useful "realism" or representation-based arguments are[n't], but honestly, one set of rocks has Circular Vision, while a slightly taller rock does not?

Yes, protection from a '+2 to attack rolls' on a model that is both Immobile and a base DEF of 5 isn't THAT important. Nor do I really care to talk about other balance/power issues with the unit itself and that giving it might be argued as "buffing" the unit. I just find it very weird and inconsistent that the Sentry Stone does not have Circular Vision.