They think that they can just extrapolate some maths to fill in the gaps and that makes the data perfectly valid and unquestionable. This is what makes the data so easy to manipulate to tell the story you want, there's so much room for tweaking the variables and weights in the extrapolation models. And that's exactly what they got caught doing in the ClimateGate emails with the "Hide the Decline" shenanigans. They start with an answer and fit the models to match, which is literally the complete opposite of how science is supposed to be done.