IMO Fender is the best example of non-standardized pickups. Just about every guitar they made had its own particular pickup; classic example is the Tele which has two different pickups, both of which are found on no other guitars (Esquire excluded). The Musicmaster bass is an example of something without its own pickup design, but that was a budget oriented instrument; no special design, no R&D expense. OTOH Gibson used the same HB's and P-90s on a very large number of models. Back to the split-P; I'd also guess that being able to arc the poles to match the fretboard radius to balance output was another consideration. He probably liked the sound of the wider coil with a bass and went that route; a HB with twin P-90 size coils would have been big and cost a lot more to make, and might not have sounded much (if any) better; and you'd still be left the string balance situation. Leo was very cost conscious.