As a full-time firefox user, I commonly find that sites have bugs, and when I boot it up in Chrome just to check my hunch, they don't manifest. It's pretty easy to figure out what happened: The developer wrote his code, tested it on chrome alone, and then called it a day. No one booted it up in another browser and even smoke-tested to see if basic use-cases work as intended.
If developers are genuinely concerned with browser diversity and not handing the Web over to Google and whatever they deem to be the next IE6, it starts with having a test plan for other browsers. Testing on only one platform is tantamount to having official support for one platform because you only guarantee your product to work on that platform in so many words. Officially supporting one platform is pressuring your users into a vendor-lock.
They - even the tech savvy ones - can't even begin to make vendor choices when the products they use accidentally lock them out left and right.