Thank you for contacting us, it seems that you may have disabled Mobile redirection for your site so when a user visits your site from a direct URL it will be taken to non-AMP version of your site.
If your AMP pages is valid and indexed by Google Search engine it will create an AMP cache version for it.
If a user searches something on google using mobile devices and the SERP shows your site as matched results and when it user click on the search result, he will be served AMP version of the page.
In case you want your direct visitors to redirect to AMP page, we will recommend turning on the mobile redirection features.