There's a couple reasons for this:
1. Bots
The internet is full of bots, like Google's bot, which are constantly crawling websites on the internet to build their search index and scrape information. We know you want to view replays of shoppers - not bots - so we filter out bot traffic to prevent your session recording limit from being consumed by them. In some cases, bots may still get through and these can show up as sessions with 0 seconds duration.
2. Bounces
A percentage of shoppers will visit your site and leave within a second or two. These visits are called bounces and may be too brief to be recorded by our app or may get listed as having 0 seconds duration.