Currently, the way Google Analytics separates users on AMP and non-AMP pages can make it difficult to get a full picture of user behavior across the two page formats.
To help make this analysis easier, Google is updating the way it counts and reports site visitors to AMP and non-AMP pages in Google Analytics by unifying user IDs when someone visits a domain via AMP and non-AMP pages.
Because the change unifies users across both page types, it will mean some reporting fluctuations. For more on the implications of this update, read the full article on Marketing Land.
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Article source: http://searchengineland.com/google-analytics-unifying-user-ids-amp-non-amp-pages-275235