Stretch in Bind Off Row

[cawhouston]

I just completed the diamond galore pattern found on this website and noticed the cast on row has significantly more stretch than does the bind off row. If I had four hands and stretched each corner horizontally I would have a trapezoid with the cast on row being longest. I used a long tail cast on and the bind off demonstrated on this website.

Is this normal and/or is there a way to insure equal stretch?