GA is filtering out bot and crawler traffic, this could explain engagements however this would not explain the conversions because i doubt the bots are buying actual products
Some people use "do not track" and other extensions to block GA and other commercial trackers from fingerprinting their browser and tracking them across sessions to prevent targeted ads. Justuno is not as known as GA and probably gets blocked less.
GA is not actually that accurate because of the way they designed it, there would be so much data for them to store that 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
Some browsers (mobile browsers ) or some software platforms and operating systems (Ubuntu for example) might be blocking GA calls on purpose or not sending them properly due to technological problems or competitive business practices