I honestly didn't get the idea behind Saturn ruling Aquarius at first, but now I sorta do.
Saturn is a planet that rules time, objectivity, rigidness, among other things. I think that does perform well in the sign of Aquarius that is about logic, reason, aloof, averse to bias. Saturn would perform more ideally in a fixed air sign.
Well, at least that's the traditional conception of Aquarius.
As I said in a traditional thread, and much to the dismay of some, Aquarius is a complex energy (like any other sign) that can manifest itself in many levels. Uranus is a planet that does actually fit well with the sign, but it's not meant to represent personal qualities of the natives that have it in such a placement. Just like Neptune rules self-sacrifice, unconditional love, high high ideals and spiritual enlightenment and its highest level, Jupiter rules expansion, fairness and compassion to all at its own level. So with Aquarius, both planets express themselves comfortably in the same sign, but on different levels.