Given that the NWS’s primary goal is public safety, there isn’t much of a downside effect of saying it’ll be 18” versus 30” because the impacts will be similar. Also, the lay person is really bad at estimating snow totals, and tend to look at snow depth rather than snowfall.
And although the ship has sailed to some degree, every time they *over*estimate snowfall it undermines confidence in the NWS forecast because the major storm was a “dud.”
Those seem like the incentives to me. They’d probably have to be incredibly confident to call for 24+