Why does four Apple Model Numbers (A1457, A1518, A1528 & A1530) have two FCC IDs (BCG‑E2643A & BCG‑E2643B)
I get the impression that Apple has done testing of different functions on multiple hardware with similarities.

From FCC:



The FCC IDs applies to different frequencies. Could be the Wifi / Bluetooth module is tested and filed under one number for a specific model, but the radio was tested on another model.

From 2643B test report:

Hints of similarities with 2643A except for the Wifi.

Usually similarity is used so you don't have to retest every configuration (memory, battery, etc) of a hardware. But the exact definition of similarity probably have to be discussed with FCC.