I’m not sure I understand what your point is. The data clearly shows a gradual increase. As with all data sets there slight variability which can easily be accounted for in inconsistent readings from the chrono or slight difference in release by shooter or any number of things. That’s why the trend is important. You can’t look at two data points and assume that is the trend. You need to look at all the data. And as many others have pointed out, the data collected in nearly all cases ( you being the only exception I think) shows a gradual increase in KE with increase in arrow weight.
What are you getting for a KE with a 420 grain arrow @ 260fps and a 450 grain arrow @ 251fps?