Originally Posted by Stez

The impression I got was, and I could be wrong, that it was more of a problem determining where the higher concentrations are going to be at a given moment in time as it is constantly on the move in a three-dimensional fashion? After all, models are just digital crystal balls.

Also the make up of volcanic ash varies from one place to another, and in time at the same place - this can affect concentration and composition of the ash overhead on distant lands.

Plenty of variables to wrestle with, so I can why one or several might have erred on the side of caution.