They have no idea when it comes to people changing their behavior in the face of different incentives.
People consume X amount of milk - if we tax it Y amount, we’ll be guaranteed Z amount of increased tax money.
Then it never happens, because people change their behavior, and they’re baffled.
On the one hand they are always trying to use the tax code to “nudge” us into changing our behavior, and on the other hand they are always befuddled when we do, and tax revenue declines as a result.
They’re bipolar, I tell ya.
Because most of the time they don't really care about collecting Z money. They just want to claim that they are going to so they can spend Z immediately. If Z doesn't come in then they "must" increase some other tax to make up the difference.