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I have no idea -- this is just a thought. Perhaps it's because Tumblr as a platform controls its entire user base while WordPress has an OSS variant that allows anyone to work with their code beyond Automattic's control.

When you value Tumblr, it's everything. When you value Automattic/WordPress you're valuing their hosting and paid packages but perhaps aren't putting a figure to their OSS offerings?

I agree that it doesn't seem to make much sense.