Why Aftermarket Parts share same OEM but different vehicles compatibility?

Question

So i'm trying to wrap my head around OEM/Aftermarket Refs/Cars relationship but things seems messy or missing data or i'm just dumb to get it.

let's say a part listed with OEM XXX

if you track the part you'll get let's say 2 variants/parts each has a slightly different list of compatible vehicules

PART n°1 / has 2 OEM's XXX1 and XXX2 / compatible with cars C1 and C2

PART n°2 / has 2 OEM's XXX2 and XXX3 / compatible with cars C1 and C4

since OEM XXX2 is shared between the two, isin't safe to say that PART n°2 is compatible with cars C1,C2 and C4 same is true for PART n°1 it becomes compatible with all C1,C2 and C4 Or is there something preventing cross reference the presence shared OEM ?

Answer 1

Quite often when an OEM has a part for a vehicle, it is specific to the year of manufacture and may differ slightly the next year. When aftermarket companies make products with a larger cross reference than what you'd see for OEM parts, one of the things you'll see them do is add in a small extra part which then gives it a broader cross reference. The difference may be an extra o-ring, gasket, or a one-use-only bolt (these are just examples). By doing this, it cuts down on the amount of different packages which are required to support a larger swath of vehicles. This makes stocking products and providing them a lot easier, which actually can save the aftermarket manufacturer money in the long run. For this reason, when things come in kit form from an aftermarket, you'll quite often have parts left over. The OEM doesn't care, because they (quite often) overcharge on what they sell in the first place. Most OEMs only maintain stock for any given part for five years past when the last vehicle it is used on is sold (well, at least here in the US it runs like that).

This is just an example of how you might be seeing the differences. There are probably other reasons as well.