There are few reasons why you might see discrepancies between the Discord members/activities tracked in Orbit vs Discord.
See below for details:
Orbit’s historical import will initially import active members of your Discord server, which are members who have sent a message within the past year.
During the historical import, Orbit will create new members and activities for any messages sent within the past year, but it will not add a member into Orbit if the member has been inactive in the Discord server. 'Server channel join’ activities will only be imported to Orbit after the Discord integration has been connected and is listening for these activities.
Members could have joined more than a year ago with no activity since, such as sending messages.
Why aren't my server joined activities showing up?
If you are not seeing any activities created when a member joins your server, then you must enable the "New Member Messages" option in your Discord server settings.
The "New Member Message" may not be enabled. Some users will need to enable this in their discord server settings in order to track "New Member Messages". This is not retroactive, so Orbit will count new member messages as activities when members join after** this is enabled in your Discord server settings.