In such situations, similar to a court of law, if it isn't a 100% verifiable that there was or wasn't pwnage, the initiator probably did pwn, just as if there is any doubt in a court the accused isn't guilty. Almost always, the purpose of a fluster pwn is to get a rise out of the one it is directed towards so that he or she who dictated the pwnage can then say "fluster pwn" in an irritating manner repeatedly.
Tommy (sarcastic): Wow, you are so good at Halo 3!!!
Kendra: Shut up!!!! You know I suck at this game. God, I just can't take you anymore!
Tommy: Fluster Pwn!!!!! (repeated slowly three times so he could gauge the reactions and adjust the response accordingly)