The experts will usually deploy a theory of the chance of their projections, an error rate etc. They will understand the nature of variability, that nothing is set in stone and that a project is only that, a projection of possible outcomes.
It's those self styled 'experts' that believe only what they believe and don't listen to other people and who are absolutely sure that what they say will come to pass.
Politicians on the other hand will use the projections to tell the story they want us to hear in order to affect our feelings and response to what they say. So if they want us to isolate they will work on the upper limits of the projections, if they want us out eating, drinking and using hospitality they will use the lower projections. Then there is cutting the data a different way, using subsets and suchlike to orchestrate the data to produce the response they need from us. All of this is out of the hands of the medical experts, they present the data, the politicians choose how to use it, which is why sometimes the medical experts will leak what they actually said when they feel the politicians have skewed the data too much.