Here is a comment from one of our customers that had to drill some new hole locations due to the short vent. We provide two rows of hole locations predrilled, but sometimes they simply wont line up. So we have additional hole locations to locate the screws to fit your roof.
I did have one that I had to work on so that the screws would grab the vent. Turns out one of the six vents was too short for the pre-drilled holes, so I had to lower the screws. It was fine after that.