If you get close to the target, use the House View in Season Manager to look at an individual performance. Maybe something will jump out at you.
I know that one issue can be that if you have a seat on hold, but have rights to break that hold, means you can sell that ticket WITHOUT breaking the hold.
For our large fall event, I had to daily check the House View for any of seats with tickets sold but also still had a hold so that the counts would not be thrown off in Analytics. I would break the hold and then on the next day the numbers would add up properly in Analytics.
Sorry, I keep thinking of one more thing.
Sometimes tickets are in an in-between state (maybe in someone's cart?) at the time when Analytics is doing its update of your data. This can also throw off your numbers. This issue usually clears up the next day (and then you notice that another perf has that issue, lol).