By default, levels in the Ternary plots are defined to be equidistant between min and max values of the predicted values.
So in your example, your predicted response values fo from 34 to 120, so the average is around 77 (3rd level displayed). For the 2nd and 4th levels, it's the middle of the corresponding remaining range :
- 2nd level : from 34 to 77, range = 43, so 2nd level is the middle of this range, around 55,5.
- 4th level : from 77 to 120, range = 43, so 4th level is the middle of this range, around 98,5.
Hope this complementary response will explain a little more how the 5 levels are distributed on this plot,
Victor GUILLER
"It is not unusual for a well-designed experiment to analyze itself" (Box, Hunter and Hunter)