Well, you've got several factors potentially in play here. First of all, it's been discussed that some forecasts are indeed not all that accurate and may be growing less accurate. Second of all, some people either don't prepare adequately or just ignore the warnings outright. This is made worse if the people repeatedly hear wrong forecasts. Third, it seems like what does or doesn't constitute a warning is up for grabs, for tornadoes and
other storms.
With that being said, the 2011 Joplin tornado truly was a devastating storm. I remember watching plenty of TV coverage. It's unusual for tornadoes to follow interstates as closely as some of the ones I've heard about in the last three years.