I've usually done the same and had the same results as you. That was why I was so surprised by the poor design on the Ventek protectors. If you read the comments on many of the HTC 10 protectors, you will see disappointed consumers like myself. Many of the manufacturers are having difficulties with curved screen edges and are avoiding lifting and haloing problems by trimming the protectors down. Unfortunately, this makes the protector too narrow to completely cover the screen.
The link you sent includes a disclaimer to that effect... see attached picture.
P.S. I have bought protectors for many phones also including Iphone 5 and 6, HTC 8, LG G4, Nexus 7, many treos and handsprings, etc., etc., etc...