Robert_Charlton - 6:08 am on Mar 12, 2013 (gmt 0)
i believe the reference to mobile search implies that these visitors are typically logged in while searching from mobile devices.
That was my assumption, which was correct as far as it went. The logged-in/ SSL issue isn't new, but my interpretation of its behavior was simplistic. There are some "paradoxes" (which is to say "inconsistencies") involved in Google's SSL implemention, plus some other twists, which have made any simple interpretation difficult.
Until recently, it turns out, Google did not automatically shift mobile users to a secure connection on Google.com, since latency issues made an SSL connection too slow. But, as observed by Ryan Jones and researched further and reported by Danny Sullivan (references below)... at the end of Sept 2012, Apple upset the cart and surprised Google by stripping all referrer data for searches made from the Safari search box on iOS6. This made searches from the Safari search box (on iOS6 only) look like direct traffic rather than search traffic. The shift began around 9/19/2012.
Google not sending any referer data on iOS default search?
September 26, 2012 - by Ryan Jones
Safari Shifts To Google Secure Search in iOS 6, Causing Search Referrer Data To Disappear
Sep 27, 2012, by Danny Sullivan
In this and the follow-up articles, Danny also comments on question of https to https referrer data. As I remember, he made the point that Google isn't using straight https, so the referrer data, which might otherwise be retrieved by an https site at the receiving end of a two-way https connection, is in fact intercepted by Google. I've seen it mentioned, but haven't read in depth, that Android 4.0 may also be dropping referrer information.
On desktop, some browsers are defaulting to https, and Google is keeping users of its various services signed in and there's increasing referrer data that's lost.