Similar plans but different treatment length for Invisalign from two different orthodontists - why the time difference?

I consulted with several orthodontists in the area and narrowed down my choices between two Diamond plus certified orthodontists. One of them claimed he can do it in 9 months while the other said 16 months. 9 months seem too promising but I also know that this orthodontist is reputable because he coaches other orthodontists on the practice of Invisalign. I'm sure both of them can deliver the same results. I just wonder why the time difference is that large between the two.