This web page looks over Ami's Astrological sign and why it makes more sense for the princess of mercury to be the same sign that rules mercury, over what she is in the story.

Virgo is only related to Mercury because it's true planet, Vulcan has not been found yet. In Fact when you look at it Mercury should not be Virgo because Virgo is in Fact an Earth Sign as well. Mercury's powers are both Ice and Fog based, basically Water and Air combined. It makes no sense for Mercury to be a Virgo which is an Earth Sign when an Air sign fits in with the fog powers much more nicely. Not to mention the fact that Virgo does not rule Mercury.

While Gemini is indeed represented by the twins, I've selected a picture to represent it's true nature. Gemini people are really two people in one, each one a unique person to him or herself. What this means is that one day you could be talking to one half of the Gemini personality, the next day you could be talking to a completely different person. Note however while I did say twins, I did not say identical twins, there is a major difference there.

Now it's hard to compare Ami's personality with Gemini because Sailor Moon was written with the idea that Ami would be Virgo, rather than the natural ruler of Mercury Gemini. As such her personality mostly reflects the Virgo and she doesn't have the Communication ability of Gemini. However both signs are attuned to knowledge simply knowledge of a different kind.

Gemini is closely attuned to personal knowledge. In other words the ability to know what to say in any given situation, as well as knowing how to react to anyone in any situation. Being a twin makes it hard for anyone to truly understand her because she's really two people in one. A concept most signs have difficulty understanding.