Something quite strange with the linear fit

I don’t know exactly what kind of problem I have.
let me explain: I wrote this code to fit the data written in the .c file down attached and the fit works (because the error bars are big, but this is not a problem because these measures are definitely ok).

But I started becoming suspicious about the error on the parameter p0 (the value is -1 plus-minus 24).
I tried to fit these data in Origin and it gave similar values of p0 and p1, but the errors on all the parameters were 0.1 times the one root calculated .

do you think there is something wrong with the code?

how the same data fitted by two different program gave two different errors ?

I hope you can somehow help me. I am sorry for the trouble
have a nice day,

strangefit.c (2.5 KB)

Your “err_lambda” values are pretty big, resulting in “big” errors of the fitted parameters.
Your “err_numeri” values are not significant here.

I suspect that the “Origin” fit does not consider your “err_*” errors.