You are very observant. That is a common left-wing tactic, to cherry-pick dates. You can pretty much "prove" any point by cherry-picking the dates.
From the Guardian article:
"In much of the public imagination, Adkisson’s [2008 -- He was pissed at Obama's campaign] and Roof’s [2015] rampages were isolated incidents. In reality, however, they were key manifestations of a larger, more disturbing phenomenon, one which has been ignored or even actively discounted by elected officials and the mainstream media – rightwing domestic terrorism.
"In the seven and a half years between those two attacks, domestic terrorism in America – acts that are plotted and executed on American soil, directed at US citizens, by actors based here – spiked dramatically. But hardly anyone noticed.
"During that time span, there were 201 total cases of domestic terrorism in the United States – almost three times the rate of the preceding eight years. The large majority of these crimes were committed by rightwing extremists – some 115 in all, compared to 63 cases of Islamist-inspired domestic terror, and 19 cases of leftwing-extremist terrorism."