Is Yahoo really having more traffic than Google? NO! But according to alexa.com top 100, yes.
First of all, keep in mind that alexa uses the toolbar to make the rankings so it may not be very accurate considering that the toolbar is not spread equally on all types of Internet users.
Then, take a look at yahoo.co.uk and at google.co.uk and note that yahoo.co.uk will redirect you to uk.yahoo.com. So while uk.yahoo.com is seen by alexa the same as yahoo.com because it's a subdomain, the google.co.uk is seen as a totally different site. The same with .de, .fr, etc. So for Yahoo, Alexa adds all the traffic from regional sites while for google it doesn't. Then think about mail.yahoo.com and gmail. Again, for yahoo it all goes in the same domain while for google not.
The same with 360.yahoo.com and blogger.com or video.yahoo.com and youtube.com. So as long as Alexa will keep counting every subdomain as being the same with the master domain, it will be very hard for google.com to outrank yahoo.com since google has it's traffic spread all over other domains.