Comm requires --nocheck-order on ~10.000 lines file?

I want to use Comm to compare two log files. The files are around 1MB containing ~10.000 lines. While testing with a small portion of a log comm -1 -3 a.log b.log > diff.log worked as expected. However when testing with the full file I got the following message; comm: file 2 is not in sorted order. When adding --nocheck-order to the command seems to work but why is this necessary? The man page doesn't really offer any insight. I just want to be sure my script outputs the correct data. I don't care in what order the logs are processed as long as it only outputs the lines that only exist in the second file.