Well I got my tarps in from that link posted not to long ago. For the price its not bad but they definitely werent 18feet. Not even the 17'6" they said on the bag But good news is the stacks are covered.
I'll have to measure the ones I bought from Menards. Mine are a light brown color, but for the price I paid($4.49 each) I can't complain.
That kind of stuff is just wrong. I would send them an email to pull one of their own out of stock and measure it. Then ask for a refund or discount.
I think you might find this very same thing with all plastic tarps. Of course, they measure the material but forget it loses length when they make the fold for grommets and stitching.