I finded that my blackberry has less spam mails then my outlook account received. I have not set any filters in my blackberry and I checked my mail server beens forward both juck mail and normal mail to
as4.bis.ap.blackberry.com. But some junk mails realy has not appear in my blackberry. Why this will happen?

It is hard to say why. Are you on BIS or BES?

If you forward your emails to blackberry, they may be filtered by spam filter they now have in BIS 2.3: Livelink - Redirection