When plotting the chart of the acoustic space of AmE vowels, we can represent F1 values on the y-axis and F2 values on the x-axis, like a chart on this site: http://sail.usc.edu/~lgoldste/General_Phonetics/Source_Filter/SFc.html
Alternatively, we can represent F1 values on the y-axis and F2-F1 values on the x-axis, like the chart on page 25: http://sail.usc.edu/~lgoldste/General_Phonetics/Source_Filter/SFc.html
It seems that the spatial distribution of vowels in the chart representing F2-F1 on the x-axis better resembles that in the IPA vowel chart, like the chart on this page: https://english.stackexchange.com/questions/676/how-can-i-practice-differentiating-between-the-%C3%A6-and-%C9%9B-sounds-in-english-pho
I was wondering if this is true. And, if it is true, why?