how to understand birth chart

So I'm still trying to figure out how the placement of all the planets has an effect on a person. I'm a sag and from the first time I read Linda Goodman's chapter on sag I could not believe how accurately it described me, it was as though she knew me better than anyone in the world, better even than myself. So I became a bit interested in astrology and started reading the signs for friends and family. But so many times I don't think they really describe them as well as mine describes me. I've done my birth chart and I have a load of sag in there so I'm wondering is this why the sag sun sign seems to describe me so well cos I'm really a full on sag? I just looked up my great grandmother's chart cos she was a Capricorn which I never felt really fitted her,and now I see that the only Capricorn placement she had was the sun. Everything else is somewhere else, so does this mean she wasn't a typical Capricorn?

How does it all work?