How can they be so sure?

Seen outside a shop advertising diet supplements:

'Can you pinch an inch? We can pinch up to 12 inches, or more!'

So, either less than twelve, or more than twelve, or twelve. And that's a promise!

Similarly (so similarly as not to deserve its own post) in a little independent video and DVD shop near me:

Sale! All films for £1.99 or less*!

*Excludes some dvds.