Why do I only get 4 turns of Golden Age for Reformation when it says 6?

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.

The length of the Golden age from Reformation no longer scales with game speed since the patch on 15 December 2010