Some APKs are autoupdated from Play Store while some are not A have option on my Play Store account - always auto update apps. Thus I see that some installed apps require update. Mainly Google apps are autoupdated in first term, but I see that even some of them are hung in the "need to be updated" list for some time. But still not Google apps reside in this list for much longer time. How much time will it take to update the apk on my device after a new version is published in the Play Store?
Some apps wont update automatically because the permissions they require (camera, storage, etc.) changed. 
Thats why you have to manually update them, so that you can accept oder decline the new app permissions.