For starters, Google Analytics (GA) is not actually that accurate when it comes down to a few things. This is because of the way it was designed. There would be too much data for them to store, so they actually down sample if you have more then 500,000 sessions in your report. You can read more about GA sampling here - https://support.google.com/analytics/answer/1042498?hl=en
When dealing with session numbers, we can appear up to 25% higher than GA. This is due to how GA tracks a session or better yet, how often they reset the session. We reset a session after 30 minutes of closing out the tab to the site/window or being inactive. From what we have noticed, GA takes a bit longer to do this.
Some browsers (mostly mobile browsers), or some software platforms and operating systems like Ubuntu might be blocking GA calls on purpose, or not sending them properly. This is due to technological problems and or competitive business practices.
If using "do not track" and other extensions to block Google and others from tracking, a customer can be hidden and go non reported in GA. With Justuno, we are capturing more than just IP and placing a cookie. so more traffic is accounted for.