I have a doubt.
Wow, a misuse of "doubt" that's close to correct. Anyhoo . . .
You don't have a "doubt", you have a "question". Your "question" is that you "doubt" that a user with only 4 writeups could be level 13 and would like to know how that happens. A "doubt" is a feeling of uncertainty; a "question" is an interrogative statement to elicit information.
This "English for Non-English Speakers" segment of "PerlMonks Street" was brought to you by a grant from the Carneigie Foundation, the letter "Y", and the number Γ(0.35).