Google can still use your web history even if you're in Incogito mode if you've logged in to any of their services. http://www.cbronline.com/blogs/technology/how_secret_is_g

EDIT: Also, your conclusion:

"Over time, these autocomplete suggetions will change, but for now it’s clear that a lot of people using Windows 8 don’t know how to do the tasks they are familiar with in Windows 7."

Why are you assuming that they're Windows 7 users who are now unable to do tasks after an upgrade? What if the Windows 8 users are people who are new to Windows?