It's entirely possible that users are:

• cancelling their open before Adobe tracking is called (while the page is loading)
• You have a large number of users who are blocking web analytics / opted-out of tracking then those values could be getting lost.
• Visits and Instances could be different because Adobe has a 30 minute visit timeout, meaning that a user could have clicked the link multiple times to come to the website within that 30 minutes, and the Visit will not recount... but the instance will.... also, I'm assuming that in Adobe you are tracking via utm or cid campagins, if the user clicked to another page of your site, then hit back (to the entry page), those campaigns are probably tracking again (unless you built in some fancy code to prevent that)

Those differences do seem a bit suspect thought... that's a pretty large extreme... you might also want to do some double checking that your campaigns are all being properly processed each time....