I have seen this in several other fields as well now that you mention it. Particularly in IS (Industry Solution) objects.
Sometimes also some fields are editable and others not.
It relates to proposals coming from the menu or from SU24. You might sometimes also see that despite merging in expert mode they remain always new status as PFCG cannot find an instance to merge them into.
If you maintain SU24 correctly, then you still get the values into the uneditable field and it is not a problem.
I faintly suspect it relates to auths from menu objects with more fields than just TCD of S_TCODE. But for me it has not been a problem for fields I care about as I have the values in SU24 -> then it works.
Cheers,
Julius