Using RooEffProd in 2D fit

Hello, my signal and background are written in the same 2D pdf : pdf1= |a1e^{ia2}(x+y) + 1|^2.
The efficiency 2D pdf is hist into RooHistPdf (pdf2).
Then define: RooEffProd pdfeff(“pdfeff”, “pdfeff”, pdf1, pdf2)
And RooAddPdf pdf(“pdf”, “pdf”, RooArgList(pdfeff), RooArgList(n));

The I fit pdf to the data.
When I free a1,a2 and n, the FCN is 99; when fix a1, a2 and free n, the FCN is 2.
why there is such a big difference in FCN

I guess @jonas can help you.

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