IIRC it’s basically a different generation of naming. 1/2 ton trucks got bigger and increased to 3/4, manufacturers labelled them 1500 (but the 1/2 ton informal naming remained), then they kept growing but the class remained.
So now you have light duty trucks with >2000 lbs payload, badged 1500, and called half ton.
So now you have light duty trucks with >2000 lbs payload, badged 1500, and called half ton.