The more relevant question in my mind would be, why would there be an expectation for two different factory castings of that vintage to flow the same as each other?
I freshened a set of 915 j heads a few years ago from a racer running in the factory stock events, that had been done by another shop.
They looked good, and decent parts in them, no porting of any kind.
The car ran decent numbers, and honestly, the heads didn’t really need freshening.
He had me flow one……….I was shocked at how good it was……215cfm.
Even taking into account the nailhead valves, it just seemed “too good”.
I scrutinized it pretty hard, saw nothing to explain it, so I flowed another port.
I don’t recall the numbers exactly, but it was very close.
More scrutiny…….more head scratching.
Then I decided to test the other head.
Tried 2 ports…..neither one broke 200.
That was a real “some castings are just better” moment.
FWIW, on both of the flow benches I’ve owned, X heads have had more flow than J heads.
