Tumblr host sites on lots of domains. If you own a domain name and have a Tumblr blog, you can configure both so Tumblr will serve your blog on that name. e.g. http://tumblr.snipe.net/ is one - the domain doesn't end in ".tumblr.com", it belongs to someone else. These probably are the ones being counted. It's very plausible that as many as 160k domains have been set up like this; Tumblr have lots and LOTS of users.
Tumblr also have a lot of subdomains, which must not have been counted. Tumblr serve every blog on its own subdomain. e.g. http://dooktrain.tumblr.com/ is a blog posting pictures of ferrets (and maybe other stuff, I didn't look). Tumblr have a LOT more than 160k registered blogs: as far as I can tell, well into the hundreds of millions. One estimate I saw put it at 300 million this July.