It's actually not weird, because English/French/Swiss/Italian bikes, despite the vagaries of tube sizes and threading, all use the same "frame cup inside seat" dimension: 30.2 mm. So any headset that provides frame cups to fit that dimension, no matter what its top cup/nut threading or fork crown race dimension, will fit the frame (theoretically; in practice, as with most interference fits, there are sometimes "adjustments" such as peening the fork crown race seat that have to be made).
https://www.sheldonbrown.com/cribsheet-headsets.html
Originally Posted by
Nate998
The measures were indeed correct. The seatpost I used was 1" in diameter, so 25.4 mm. I managed to measure the head tube diameter and it is 32.5. The fork steerer tube diameter is 25.0 mm.
This is weird because I once replaced the fork with a 1" steerer carbon fork and it was compatible and free of any problems, obviously I used another headset for the carbon forks.