I suspect (can't know) that the issue might be the result of unthinking frame prep, by a rank amateur mechanic.
Consider, the head tube ID (when round) is matched to the head cups. Actually slightly under allowing reaming to an exact ID for a proper interference fit (under 0.5mm). Even if it were ovalized afterward, the circumference would be unchanged, so pressing the cup in would restore the correct fit by forcing it round.
However, consider if it were ovalized BEFORE someone opted to pass a reamer through. The reamer would cut from the narrow side, but, obviously, not add that metal back to the long side. The result is that the minor diameter is now correct, but the long side still too long, causing EXACTLY what you describe.
The above assumes a relatively new bike. If this were a well used mtn bike, there's the possibility that repetitive impacts caused bellmouth of the lower head tube, but that could only appky ot the lower cup.
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