It might not strictly answer your question, but there are statistics published for the number of visas issued every month.
In February, Montreal issued 20 CR1 visas and 78 IR1 visas. January, around 170 total, December around 120 total. November around 140.
My guess is that people are DQ'd by the NVC in order, but interviews are scheduled according to the capacity of the consulate which seems to be around 100-150 visas per month. Like I said, just a guess...