Tesseract giving diffrent results for the same image when "tessedit_char_whitelist" is set and ocr is done

Question

I am using tesseract to find text in scanned image,

I am trying to set "tessedit_char_whitelist" by calling as follows, so that only alphanumeric search is performed

tes_obj.SetVariable("tessedit_char_whitelist", "abcdefghijklmnopqrstuvwxyz012345789ABCDEFGHIJKLMNOPQRSTUVWXYZ");

When I try to compare results between multiple searches on the same image, I found different results. What might be the reason for different results.

Regards, Amrut