Sometimes you just get one from a faulty batch... or if you buy in bulk you sometimes get a bad batch. I'm not sure if the price you pay is an guarantee either, as I can recall buying a 12-pack of Michelin tubes many moons ago from an Etailer and after the third one developed a mystery leak along a seam within the span of 3 hours, I borrowed a tube to get home and returned the lot.
Since then I've simply bought the 10 pack lessor-known brands when they go on sale at Jenson, Performance or where ever and they've worked just fine, particularly given the $2/tube cost.
Obviously, older tubes that have been folded up in a box for a few years or in harsh conditions can also be problematic so you need to balance carrying your own inventory against your consumption habits to make sure you don't create your own problems.