This isn't related to prototype armor or anything, it's just a side effect of the fact that Best States are not recorded every turn. They are only recorded occasionally, since it's a whole bunch of extra calculations and not really vital stats to maintain constantly, just a snapshot. So you'll notice if you attach something and do a stat dump (or the run ends soon for some reason) the values that changed recently wouldn't be reflected.
(The build type system works similarly, although for slightly different reasons: because people are frequently swapping parts in and out, and we don't want the build categorization to potentially be swinging back and forth all over the place, so it doesn't even record anything until a build has reached a "stable" state, defined as not having attached or swapped any parts at all for a given period.)