CHEKOV (OC): I'd swear something that was there sir.
KIRK (OC): What did you see?
CHEKOV: For an instant ...a scout class vessel.
KIRK: Could be Grissom. Patch in the hailing frequency. Grissom, this is Enterprise calling. Please come in.
Whether she really is a scout class vessel as of the 2280s is uncertain; Kirk isn't quite convinced that Chekov's observation of a "scout class vessel" should be explained by it being the Grissom, but rather just says, with doubt in his voice, that it "could be" that ship.
I think you are reading too much into this. Ken Ralston said it's a "Scout Class vehicle" which matches the onscreen dialogue.
The one thing that hasn't been mentioned that, is that the Federation Survey
Vessel USS Grissom
is supposedly a member of the Scout Class.
What Chekov actually sees is Kruge's vessel, which obviously does have characteristics of a scouting "bird" but in contrast to the characteristics of a surveying "turtle" which could better qualify as an analogy for the Grissom,
Too bad, we do not learn what it is Chekov is seeing on his displays (mass and/or volume of the UFO?) but I think it may have something to do with size.
The Bird of Prey has a length of 360', the Grissom
of 395'. Theoretically "Scout Class" may just refer to size, opposite to larger vessels that would qualify for the Destroyer or the Starship Class.
Inevitably, these classes would have to be adjusted on a regular basis. What could have qualified as a starship in the early 23rd Century has "become" too small by the late 23rd Century to still belong to the Starship Class.
This could also explain why ships of the Constitution, the Miranda and most likely the Constellation Class (e.g. Stargazer, Hathaway
), too, belonged to the Starship Class in the late 23rd Century but no longer do so in the 24th Century, next to the larger starships of the Ambassador and Galaxy Class (considering that the Hathaway in "Peak Performance" is referred to as a 80-year-old "starcruiser" and not a "starship").