Static and dynamic analysis. From the article:
"The Tax Foundation found that Trump's proposal would lose around $10.1 trillion when accounting for the potential economic growth that would be spurred by the tax changes.  The $12 trillion figure comes from more traditional scoring methods, akin to those employed by official federal scorekeepers."
In fact, they almost are wrong all the time. No one, not even Andrew Mellon, would have predicted his tax cuts in 1921 would have led to 1.6% unemployment in 1926. Utterly unheard of in the modern industrial world.