Very interesting.Why I run 11/13.
I flipped to 11/13 for two weeks the first, and second time I had a plant that spent 3+ weeks under 12/12 and was filling the room rather than starting to flower. The first time she spent 6 weeks under 12/12 and yielded as much as the other 3 girls in the room. The second time she spent 3 weeks and had a normal yield. The third time it happened, I only waited 2 weeks, and I left the flower lights at 11/13.
Since going to 11/13, I never had an issue with flowering taking more than 6 days.
If it makes a difference, these were three different hybrids from three different sources under 600W HPS. One of the hybrids was a clone of a mother that had flipped under 12/12.