Individual rank is calculated by ranking each player in each statistical category (i.e. K/D, K/S, etc.), then adding those ranks together with the person with the lowest total ranking at 1, the next lowest at 2, etc.
Overall rank simply adds together the fighter, bomber, attack and vehicle ranks and then goes through the same process.
According to the OP this process was working for all other categories except fighter rank. I would have a hard time believing that HT thought that fighter rank was sooo special that he would develop an entire new code and process to determine what it was, therefore I think this was nothing more than a bug.