Right, each Sphinx is as likely as any other. I'm saying what the odds are that you will have a certain outcome after x number of tries, taken -as a whole-. Clearly any given Sphinx battle has the same chance of giving the Old Helmet drop - 1% per Sphinx.
Another way to think about it: if you've already fought 400 Sphinxes, and you DIDN'T get the drop, it changes nothing regarding the outcome of your next battle.
However, if you are still GOING to fight 400 Sphinxes, every Sphinx still has the 1% chance of dropping the Old Helmet apiece, therefore you can say what the odds are that you will get the drop sometime during that 400-Sphinx run.