You don't have to guess, its a specific article by the NIJ that was noted.

It was also pulled by the DOJ because they didnt like what it said right after Kirk was shot.

It was, however, very quickly archived.

What NIJ Research Tells Us About Domestic Terrorism

You can look up NIJ methodology if you are concerned. I would generally agree with you that it is undercounted/underreported and there is a lot more conservative violence than we know about.

Conservatism always looks to create an outside group it uses to exploit and blame their self-derived problems on. See currently blaming the immigrants for taking jobs and benefits away from working class Americans when it is really our existing system that does this in the name of profits.