Guy giving you his number?

Its just a way to avoid putting the girl in an embarrassing situation that she might shy to say no and give her number while she doesn't like to, so the boy give his number and if she is interested then she will call back, some way of showing high respect to the girl and high dignity

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.

Example
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?

Although I agree a small bit with Deline's answer, I think it's more polite nowadays. With so many nut cases out there and the stalking issue, giving the girl the option to initiate further contact with someone they met in a public place, such as a bar, is a safe practice.

It usually means he's to chicken to ask for yours. A guy asking for your number is usually a guy that's far more confident.

Do with that information as you please. Doesn't mean the chicken guy isn't more or less interested than the other guy.but he's probably a little too intimidated to handle your rejection.that is you choose to reject him.

he is letting you know he would like you to call him if you are interested. he probably figures if he gives you his number and you call then you may be interested in him.