A lot of Tumblr pageviews are dashboard based, whereas WordPress.com's tend to be much more front facing. Also, I imagine the amount of time a Tumblr user spends per page is less than those on WP - just by typical nature of the content. Just guessing, but that's my assumption.

Mullenweg has said before he pictures Tumblr as more of a social network than a blogging platform. I wish I could find that right now. Either way, WordPress.com is definitely learning some things from Tumblr. The new wp.com dashboard and quick posting mechanism is enough proof of that.