I've noticed that there is some sort of spectrum/range into which each type can be placed. Therefore, in one person's subjective typing system a person can be LII, yet fairly similar although different types in someone else's - e.g., EII or ILE.
It might even have to do with subtypes.
An Fe-ESE might be perceived as "EIE" by some LIIs (those who place greater emphasis on Si), or an Si-ESE as an "SEI" by others, depending on their subtype. I think if one were to put the pieces together, there would be an overall framework into which every type can be placed that is consistent with this notion of a range of subtypes and their resulting relations; the nature of socionics could even be so strange that to one LII an "SEI" activation partner is objectively an IEI (their dual is SLE), yet there is still some objective overarching mathematical scheme to explain human behaviour that fits reasonably well with Model A.
On top of this, there are probably other key information elements which are hidden within the individual IMs. (E.g., consider an extremely warm, caring E2 ESE versus an outgoing, friendly E7 ESE.)