Immelmann "Kill" Total

I have become interested in the air war during WWI over the last couple years-especially a handful of the aces. One of the aces that interests me is Max Immelmann. In the research I have done concerning him, different sources record different numbers of "kills" he had obtained by the time of his death. Some say 15, 16, and 17 (like this website says "17").

I was wondering why this is the case. I have always thought that the German's were big on keeping accurate records about everything. One thought that crossed my mind is that kills #16 and #17 were said to occur on June 18, 1916, the day that Immelmann died-I would assume on the same flight that he was killed. Could this be the cause of the difference found in sources-that he himself didn't report them and that they wern't certainly confirmed or somthing? Also, what is the believed accurate number of kills that he recorded before his death?