Right now the weather (look at a MSLP chart) is lots of little bits of muck all over the place, moving about fairly randomly, and this kind of stuff cannot be forecast well.
You could stick 10 forecasters in a room with the same 3D model data and each of them would draw the troughs in different places.
OTOH if you get a big front, then a gap, then another big front, which is the more usual situation here, that is much easier to forecast. Well, they often get the timing a bit wrong but the forecast weather will definitely turn up.
There is also some subtle shorthand involved:
Prob 30 means they think the stuff will likely not happen but it just might and they need to cover their ar*e. I know an airline pilot whose airline SOP is to ignore all "prob 30 tempo" forecasts.
Prob 40 means they think the stuff will come.