Whenever I get Reformation it says it will cause a 6 turn golden age, but invariably it only adds 4 turns. Could this be because I have built Chichen Itza (which increases the length of GAs by 50%) and it is bugged and doesn't always or could there be another reason?
I've just tested it and the Golden Age you get from Reformation is unaffected by Chichen Itza or the Great Person shortening.
It is affected by the Persian Unique Ability, which increases the length by 50% to 9 turns.
It is not affected by the game speed, reformation alsways gives you a 10 turns Golden age.
According the the game manual:
Using a great person to initiate a Golden Age shortens the length of the next "Great Person-based Golden Age", but I assume it also affects other methods of initiating an immediate Golden Age.
Did you use a Great Person to initiate a Golden Age recently?