The answer is simple: Because they're people. Humans are unpredictable because we're complicated, and no one human being is exactly like any other. Most of us are a mystery even to ourselves, let alone to anyone else.
What can you do about this? Well, the first step is to stop thinking in generalisations. If, every time your girlfriend, partner or whoever does something you didn't expect or don't understand, you just say 'Oh, that's typical of you women', you are guaranteed never to get to understand her, or any other woman. And she probably won't make the effort to understand you either.
So, next time this happens, try asking 'Why did you do that?' Not angrily, just seriously. You could even say that you find it hard to understand, but you would like to. Get her to explain, if she can. This is a good first step towards the two of you starting to see each other as individual human beings and not just 'typical man/typical woman'.