Buck converter - AC small signal equivalent circuit

I have 2 different type of AC small signal model of a simple buck converter.

Although both of them represents a simple buck converter, why are the two graphs different from each other?

Source: https://ninova.itu.edu.tr/en/courses/institute-of-science-and-technology/132/elk-506e/ekkaynaklar?g45747

Source: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=124585