But some people just hear of Naruto Shippuuden and they have no idea how it is spelled. So when they search, they need to type it out, instead of just saying it. The easiest way to spell it would be Shi + Pu + Den = Shipuden. But from my observations, those who search for Sipuden instead of Shippuuden are from Asian countries where "Si" exists, so there you have it: Sipuden.
But who knows, maybe locally there, the series might have been translated from Shippuuden to Sipuden? Or could it be just a typo with the "H" missing?