Suppose you have real data in a range but want to prove bogus data. It can always be done by introducing a fudge factor.
Your real data will produce a line on a graph. The bogus data would be another line totally unrelated to the first. How do you make them match? By introducing a third fudge factor line that is the difference between the first two and adding that to the real data. The equation that produces that fudge factor line might be complicated or not, but it always exists.
But what happens when you go beyond the range of the initial graph? The illusion is broken because that fudge factor line is unlikely to produce the same bogus result outside the original range.
This is climate science models in a nutshell.