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Bullet seaters contact a portion or point on the ogive not the point of the bullet itself. Thus measuring with a comparitor, which does the same, will give you a much better idea of the consistency of the seating depth than measuring off the point.
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Ok, the comparator contacts a point on the ogive, in the same manner the bullet seating stem does, but is it the same point? The comparator is dependent on consistent ogive at the point of contact, bullet to bullet, so, a bullet that is slightly "fatter" or "thinner" at that contact point is going to give you what kind of reading??
My point about measuring over all loaded length is for comparison between the cannelure showing and cannelure hidden ammo the OP has. One must first discover a difference, before you can decide if it is significant, or not.
IF, for example, the rounds with the cannelure covered are .0Xx shorter than the other "normal" rounds, that is something you need to know before you can figure out why.
Sure, there are lots of possibilities, inconsistent bullets, or seating depths, or case lengths, or possibly things we haven't thought of yet.
It LOOKS like slightly deeper seating depth hiding the cannelure under the case mouth, but, is it?? Or something else that produces the same visual effect?
Bullets with slightly different locations of the cannelure (across the same lot) can produce that effect and still have the same over all loaded length.
Another possibility is that the hidden cannelure rounds have cases with poor neck tension which allowed the bullet to move slightly deeper into the neck than the other rounds.
Also consider that GI ammo is loaded "good enough to work" not "best possible ammo", and this might simply be an example of a bit more slop than usual.