I always thought there was an unwritten rule about whether and man gives his number or asks for hers.
Ask for her number
If you are sufficiently familiar with the girl. I.e. know each other though friends, have talked for a long time at a party.
Your confident that they she is interested in you.
Give you own number
When you've only meet the girl for a short period of time, but have got on really well. Your not confident that she is interested in you, she may just be friendly, she may even have a boy friend or like girls.
I was in an opticians and was getting on really well with the girl helping me wear contact lenses for the first time. We had a laugh and were lightly flirting. I was talking to her for half an hour. I wanted to see if she wanted to go on a date. I could have asked her for a telephone number, but if she had a boyfriend this would have caused an awkward situation, that would have change the mood from friendly flirting to a difficult shoe staring. Instead I gave her my number just as I left. It was quick, she was flattered. She never phoned but because it was such a brief encounter I wasn't upset.
Maybe its just where/how your meeting men?