It could have have been a careless installation.
A lot of combustion air may have been bypassing the filter.
I found a comparable situation when I bought my '65 Comet with a 289 CI V8. The gasket between the metal air filter housing and the carburettor had shifted and some combustion air had obviously been bypassing the filter. There was even a bit of dust on top of the carb as evidence of air leakage.
That engine didn't last very long. I bought it with less than 40,000 miles and the engine was toast by 80,000 miles. By then it was using quite a bit of oil and smoked on deceleration. It had new valve seals too (redid the heads part way through for a burned valve). It's possible the odo had been turned back and there were a lot more miles on it than the odo showed, or maybe the dust that got into the engine had something to do with it, or maybe it was something else entirely. Whatever the reason, that engine didn't last very long. That was in an era when you could generally expect to get 100,000 miles out of an engine, though not a lot more. Even so i always considered that poor service from one of Ford's premiere engines of the era.