I have a theory that the degrees of the zodiac are related to their rulers only accidentally and by affinity. It seems more logical to me that Aquarius is a wintry portion of the zodiac that causes cold weather that is similar to Saturn rather than Saturn mystically ruling it and it itself causing the cold weather. Thus the rulership is only a useful metaphor, since the placement of the ruler influences the same weather, and ambient at the moment of the birth, whether favourably or unfavourably. The same should be true in the case of the fixed stars, no one should be baffled by the fact that some distant star exerts similar influence to that of an admixture of the planets in the solar system - for example Sirius has an effect similar to Jupiter and moderately Mars.
I believe that Ptolemy is saying the same thing in the following passage:
''
and let us remember that in the combinations, again, we must consider not only the mixture of the planets one with another, but also their combination with the others that share in the same nature, whether they be fixed stars or signs of the zodiac, by virtue of their affinities with the planets, already set forth'' -
http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Ptolemy/Tetrabiblos/2B*.html#8
According to Bouché-Leclercq there is some evidence to suggest that some philosophers thought that Saturn itself causes the wintry weather of Capricorn and Aquarius, which he is quick to berate, though I believe their investigation of the affinity may have been way more subtle than that.