Feedback compensation on flyback PFC

[Fluorescence]

Hello,

I write concerning a flyback PFC stage with L6561.

I refer to Figure 16, page 18 of the following app note...

http://www.st.com/stonline/products/literature/an/5956.pdf

There are feedback compensation components on both primary and secondary sides. Is this unusual?
I would have thought the gain and phase margins could have been set on just the secondary side alone.

here is the L6561 datasheet...

http://www.st.com/stonline/books/pdf/docs/5109.pdf

Pin 1 is inv input of error amp
Pin 2 is output of error amp