Why does the gear oil 75w-90 bottle say it conforms to SAE 10w-40 15w-40 motor oil?

75-90 is commonly known as GL-5 rear diff oil. also used in certain standard transmissions. 10-40 15-40 is usually used in Diesel engines. it's thicker than regular motor oil. I have no idea why it's labeled that way tho. I have used the 75-90 in small amounts as an additive to my engines in the past to thicken up the oil and slow down leaks. Why does the gear oil 75w-90 bottle say it conforms to SAE 10w-40 15w-40 motor oil?
Where does it say that at?

bloom-