"plotPull" method of "RooMCStudy"

Dear RooFit experts,
I have a question on “plotPull” method of “RooMCStudy” class.
I wonder why when I ask to “plotPull” to fit the pulls with a guassian, the fit often come out not correct, while if I fit it “by-hand” using the “Fit Panel”, the fits come out perfect.

I’m attaching two png files to this post:
(A) “BadPulls.png” which contains the output of the toy-MC studies from “plotPull” in blue
(B) “GoddPulls.png” which contains the output of the toy-MC studies from “plotPull” in blue AND the fit to the pulls by using the “Fit Panel” in red

As you can see the “Fit Panel” fit is able to find the good gaussian right away.

Please, let me know if there are some options I can use to allow the “plotPull” method to find the right gaussian fit.
Regards,