I have no idea why that list includes some incidents while omitting others - most notably of course, the Tucson shooting that very nearly killed Gabby Giffords. Or the Jonesboro school massacre. If you look at the link below you'll find 44 massacres in the US since 1980, and that list too is missing some that do appear on others, like the Oikos thing this year. I haven't checked if all those massacres happened with guns, admittedly, but odds are the vast majority did.

I don't even necessarily know if the US has much more of these things per capita than other countries, either. I was just baffled by your very low number, and my quick and lazy searching seems to confirm it should be rather more.