Hi anne3,
This thread is a great opportunity to go deeper into astrology. It's entirely possible that you have run into many people in your life whose outer tendencies and actions seem to represent the opposite sign from their sun sign. But if that's the case, it's not because the general rule is that we gravitate towards the opposite of our sun sign.
Instead, it probably suggests something about the natal chart of each of those individuals you've met - something we won't know until we look at the chart.
The answer is not that we move away from our Sun sign but that people are more complex than mere Sun signs and the whole chart reflects that.
For one example, flapjacks's post above. (Quick advice, always pay attention to what flapjacks is saying, even when I disagree with her.

She's always onto
something... So too a whole
host of names on this site).
Second, Many astrologer's will emphasize that when we know a person on just a day-to-day level we often experience their Ascendant - which is associated with a person's personality - more than then Sun energy. (So, some of the people you're thinking about could have their Sun opposing their Ascendant).
Third, I could offer myself as another example. I have a glob of planets conjunct in Aries - Sun, Moon and Venus. Yet when people I know talk about me they treat me as if I'm a lIbra. That is, they think of me as a person who primarily possesses the traits of a Libra (if they knew astrology, they'd guess I was a Libra) in that I tend to be all about negotiating, mediating and reconciling problems and issues. Why am I like this? Well, one response: Those Aries planets are in the 8th house, related to Pluto, while Pluto is the 1st house, related to Aries. And my Pluto, as it turns out, is in Libras - in opposition to that glob of Aries planets, actually. Now, a few months ago I read Steven Forrest's book on Pluto and was shocked at how prevalent his three pages on Pluto in Libra was to my life. Pluto is a generational planet and so doesn't always affect individuals so obviously, but because my Pluto is in my first house (and I hear the voice of graybeard in my head saying "And it's the leading planet in the locomotive pattern of my chart"), it really becomes very obvious in my personality.
That said, if you had me pegged as a "Libra" you'd be absolutely wrong. I am profoundly Aries-like. And I have a number of close aspects with my Neptune, and my chart's ruling planet in Pisces, which shows up in me just as much if not more than the Aries. But I am an Arian/Neptunian person whose outer personality (1st house) is all bound up with dire needs and drives for human reconciliation. People see me and primarily they see the Pluto in Libra - much, much more than even my Virgo Ascendent!
Sorry.. that accidentally went into self-analysis there. But the point is this: The more you look at whole charts and start putting the pieces together (as I just did with my own) the more you begin to see why Sun sign astrology is misleading - and exactly why you're bound to run into people who seem to be and behave very differently than their sun sign.
