What's the difference between the Apple Part Number (APN) and the Part Number? I'd like to know what's the reason for Apple using two different nomenclatures for parts.  

For example: an iPhone 6s battery has these two ways to call it:


Apple Part Number (APN): 661-00033, this is the one that is printed on the part itself.
Part Number: 661-04581, this is the one that appears in GSX.


Why doesn't Apple use only one?
Here's what I inferred from https://thebookyard.wordpress.com/2014/04/09/apple-part-numbers-explained/ :

The APN part number is the part number that refers to a certain version and model of a component. Apparently, this can change based on minor revisions, manufacturer changes (sometimes Apple uses multiple manufacturers), and other things. You could compare this to a lot number of a medicine. Each time a drug manufacturer makes a small change to the medicine, whether it is color or packaging, for example, a new lot number is assigned. In the event of a recall, they can use the lot number to identify the specific batch that is affected.

The part number that appears in GSX is a part number that refers to the actual part. This normally stays the same through minor revisions. To continue with the medicine analogy, this could be like the UPC barcode on the bottle. The medicine is still the same (for instance, if Tylenol decided to make a change to their cherry flavor), but slightly different ingredients.