Steve, I think the width of the bottom border can vary, with some many being narrower. Maybe this was done a bit wider, in case some play dates and times wanted to be added in by a theater owner (?) US inserts measure in at 14 x 36 inches. The top border, too, is a bit wider than either side.
When i asked her how and why she picked this (she got it from an online seller), she said she liked the imagery and that it looked like a fun movie. All the reasons that collectors can go for a certain poster, too!
And I like that it is drawn art rather than cut and pasted photos.