you lay want to replace "%mkt_opt_out_prefix%" with your landing page subdomain, but AFAIK, this is not the cause of discrepancies anyway.
Otherwise, your code look OK.
Everything that I've read says that the unsubscribe total in Email Performance Report is counted upon the unsubscribe link click.
It's filling out the form from the tracked link that consitutes an unsubscribe.
We have noticed this as well. We use this set up (I changed the names).
There are a few variables that are tough to account for.
1) If you sent that record multiple emails that could make the numbers not line up.
2) If they unsubscribed from another email if would change the numbers.
The more specific you can get with the filters the more accurate the number will be (Dates, Emails, Links, Pages, Lists, etc).