How are the starting values generated when using pre-defined functions in cftool?

I'm using cftool to find what function best describes my data. I noticed that if I use the same function as that of e.g. gauss3 as a custom function, the fit is worse than when I use the gauss3 predefined version. This difference is due to the different starting values used, which were different in the mfiles I generated in both cases. Using the starting values generated for the predefined function case in the custom made function did results in the same fit.

How are the starting values generated when using pre-defined functions? This is apparently different for custom functions (for which the values were completely off). I know that I can manually adjust the starting values, but that is not an efficient option given my complete data set.

Thanks in advance for any help on this!