This is a pretty complex problem so Google outlines two main reasons:
1: Page Hierarchies - namely how you order pages or sub-pages on your site may determine how many Pageviews, Sessions, Unique Pageviews, Users, and Entrances have occurred, each with its own tally.
2: Times and Dates - if someone is on your site at 11:59 and 12:01 those may count as two visits. Alternatively, you may have it set up so that 8:59 is one session and 9:00 is another, though no new pages were selected the user could be counted twice.
Here is the full explanation from Google on why this is the case: https://support.google.com/analytics/answer/2934985?hl=en
Updated over 1 year ago