Comparing Jetpack's post view statistics to my own server logs, I found that Jetpack underreported views by a factor of five or more. I wrote up the details in this post.
As explained there, I can't see how the obvious explanations -- my own logged-in views, known spiders and spammers -- could possibly account for this discrepancy. So is this a bug? A deliberate non-continuous sampling of views to reduce the load on WordPress.com? Something else that I'm missing?