The lone gunman thing, where someone flips out and shoots up a Navy base, or bombs a government building because of a conspiracy theory, is distinctively American.
A conspiratorial explanation: perhaps the data has been manipulated to downplay terrorist coordination in America. There would be a couple reasons to do that, depending on the audience. For ordinary citizens, a collection of nutcases might be less scary than a dark underworld of coordinated cells. For terrorists and wannabe terrorists, a collection of uncool loners might less glamorous (as you've suggested too) than a Team of Subversive Evildoers.
But this doesn't explain why the same manipulation hasn't been done for other countries.
It looks to me (admittedly mostly from the outside; I don't live in the US, though I travel there sometimes) much more as if the government has played up the risk of terrorism by organizations like al Qaida than as if it's trying to make it seem less threatening. (It seems like there's less of this under the present government than its predecessor, which cynically would make sense because there's some evidence that people's votes tend to shift "rightward" when they are afraid for their lives.)
Yesterday I was using the Global Terrorism Database to check some suprisingly low figures on what percentage of terrorist acts are committed by Muslims. (Short answer: Worldwide since 2000, about 80%, rather than 0.4 - 6% as given in various sources.) But I found some odd patterns in the data for the United States. Look at this chart of terrorist acts in the US which meet GTD criteria I-III and are listed as "unambiguous":