Thanks for the answer Bruce!
 
I think one thing I did not understand about the single fit is that it tries to use all standards (although some of the standard's weights can be equal to 0 in the results), which can be detrimental to the fitting. Thanks a lot for the precisions, things are clearer now!

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Adrien Couet
Graduate Student
Dept of Nuclear Engineering, Penn State University
+1 814 865 9709
axc1019@psu.edu