I think I pulled it from wikipedia, but a number of sources say the same thing--without citation (when did not providing citations become a "thing"?). Mass shootings have certainly increased dramatically over the past two decades, but surely not that dramatically. At the same time, Columbine was a rather unprecedented event. Still... My guess would be that it comes down to how they are defining mass shootings, but also keep in mind that most sources exclude gang-related shootings and acts of terrorism or politically motivated violence from the category. Not that that even clears things up much--I mean, were the participants in the poll privy to the varied definitions of mass shootings, and all the particular exclusions, and which ones the pollsters were adhering to?
Regardless, I think 7 percent still rather a high figure--and that 2 percent having sustained injuries especially so. But as gmilam noted, it's not hard to believe that that number have witnessed any sort of shooting; in fact, I'd guess that the percentage of Americans who've witnessed a shooting in general to be much higher, given that some years see as many as 50 thousand shooting-related deaths.