As my brother and I walked up to Alum Pot the other day I pondered the apparent anomaly of why the dry valley below Alum Pot leaves the surface gash at the same horizon as the present day Alum Pot Beck spill over into the pot.

Such similar problems exist elsewhere.

Surely if Alum is of pre glacial / glacial origin  then there's been plenty of time for Alum Pot Beck to cut a canyon deeper than the outflow valley.

Can anyone explain why the horizons are the same?

That's an interesting question; could it be simply that the stream that at present falls into Alum Pot has nothing whatsoever to do with its formation, and is simply there by chance? So the pot was there before the stream, and somehow the stream has found itself falling into the pot in the relatively recent past, before it had time to do much downcutting, upstream of the open shaft?