Many browsers, especially mobile, now have a feature that will strip away "excess" and show the reduced version of a page to the user, presumably for faster viewing. It's clear to see that browsers are developed for the user and *not* the website owner.
Mobile Chrome on Android* (the world's most used mobile OS) displays across the bottom of the screen the option to "Show simplified view" even on fast loading pages. This chosen... gone are all ads, all navigation, most links, most CSS enhancements, most scripting, etc.
This is done from the mibile Chrome browser's cache and additional requests to the page are not needed - thus there are no identifiable attributes added or changed to the UA string, which makes it undetectable just how many users are doing this while at our websites.
*
Added in Android 8.0 [edited by: keyplyr at 9:56 am (utc) on Oct 8, 2018]