Now that more than 50% of all searches happen on mobile devices, Google's Gary Illyes says that it makes sense to have a separate, mobile-only index. Actually, they've been working on one for a little over a year. A new article from The SEM Post details some of the difficulties Google's been having. They point to how different a mobile-only index might be.
For example, "Google found there are fewer linking rank signals on mobile pages when compared to the desktop equivalent." That's part of the reason why pages rank differently on desktop and mobile. There's also the issue of content - mobile versions of some pages are truncated. Overall, there's just less content on mobile devices.
What is Google's goal with a mobile-only index? Are they just trying to make desktop and mobile devices show the same SERPs, or will the two always be different?