Have seen this in versions of Adobe Illustrator since cs4 to cc 2021
Currently running on Mac OS 11 Big Sur.

When you click with the graph tool the graph generated does not match the dimensions entered. I have had results that vary from 1 to 2 points smaller but not always the same though always 1, 1.5 or 2 points. If it is one of the circular graphs it is even worse with the graph being 12.5% smaller plus -1.5 points. I have clients who request exact graph sizes and it takes 2 or three attempts to get it right without having to manually scale and distort the graph to fit. First graph is not always, though sometimes, the same as second graph in the same document with same settings. After second graph they all seem to be the same amount smaller than specified. A file with two graphs created at 200 x 150pt is attached to illustrate what I mean. Created first graph, drew the grey rectangle snapping to the points of the graph area then created second graph with IDENTICAL settings and again drew the grey rectangle behind that one. difference is 1 point smaller on the first and 2 points smaller on the second. Usually further graphs are then consistently 2 points smaller but always have to check to be sure.