The trick you're missing is probably the one being played by the carrier.

It's not uncommon in all types of travel to be able to get prices for products they're not actually selling. There are a few cases where I think it's a deliberate bait-and-switch but most of the time I think it's a system glitch.

In theory you can occasionally be viewing a product/fare that's available but while you're deliberating on it, someone else books it out from under you. And that's probably what customer service will tell you is happening, but of course it's nonsense when it happens over and over. You just have to accept that the fare doesn't really exist. Sorry.