Are you analyzing the list of "Notable Domestic Terrorist Attacks" on that page? Which has already been filtered by some criteria of notability?

A more complete list is actually prompted at the top of that section and is https://en.m.wikipedia.org/wiki/Terrorism_in_the_United_Stat....

However, you've possibly read that already since you're 41% number appears to be sourced from that page and is specifically talking about deaths and not events from 9/11/2001 to 2017. That 41% is heavily influenced by the deadliest event which was the Orlando Shooting, and if you look at the overall picture, 73% of events were perpetrated by white supremacists.

Honestly, directly reading the GAO study and the other, more recent, studies is a lot more illuminating and illustrates the growing issue of white supremacy and far-right political violence.

The link you referenced[0] also includes this:

   A 2017 report by The Nation Institute and the Center for Investigative 
   Reporting analyzed a list of the terrorist incidents which occurred in the US 
   between 2008 and 2016.[27] It found:[28]

   115 far-right inspired terrorist incidents. 35% of these incidents were 
   foiled (this number means that no terrorist attacks occurred) and 29% of them 
   resulted in fatalities. These incidents caused 79 deaths.
   
   63 Islamist inspired terrorist incidents. 76% of these terrorist incidents 
   were foiled (this number means that no terrorist attacks occurred) and 13% of 
   them resulted in fatalities. These incidents caused 90 deaths.

   19 far-left inspired terrorist incidents. 20% of these terrorist incidents 
   were foiled (this number means that no terrorist attacks occurred) and 10% of 
   them resulted in fatalities. Two of these incidents were described as 
   "plausibly" attributed to a perpetrator with left-wing sympathies and caused 
   7 deaths. These are not included in the official government database.[15]

[0] https://en.m.wikipedia.org/wiki/Terrorism_in_the_United_Stat...