This is not surprising. Mobile is still pretty early and hasn’t scaled much. I bet search accounts for a huge piece of the mobile revs too. You can talk to any big agency out there to figure out that this is the state of play not only at Yahoo but almost everywhere else.
Because, according to sources inside the company, direct mobile revenue hovers only around $125 million annually, mostly from search revenue on mobile devices. While users are consuming lots of Yahoo Web pages on their phones, which could technically boost the sales numbers higher, rendering most desktop-created ads on mobile is not the same thing as the significant mobile revenue Yahoo needs to generate.