The reason becomes clear when you think about the underlying levels equation which gives rise to the linear equation
y = -x
The levels equation is
Y = 1/X
(or, more generally Y = C/X where C is any constant).
Then a 20% increase in X corresponds to only about a 16.667% drop in Y since 1/1.2 = 0.83333.
For more details, including a simple TAB file you can run to help you understand how GEMPACK works in multi-step calculations, download and read either a Word document or a PDF file.
You may be interested in a related example.
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