Maniphest burnup report incorrectly counts "All Time" opened and closed tasks. It seems like it considers closed tasks migrated from Bugzilla to be still open.
For example, for MediaWiki-Page-editing, https://phabricator.wikimedia.org/maniphest/report/burn/?project=PHID-PROJ-hd4zy7ho6fpqtllay7jm reports 1,422 opened and 66 closed tasks, while there are in fact 310 open and 1,103 closed tasks. (And why these numbers don't match each other is yet another mystery.)
It seems that tasks migrated from Bugzilla that were already closed there do not have any "event" marking that it was closed. For example, T116178 has an entry "TheDJ closed this task as "Resolved".", while T2015 has nothing like this. I think these should be backfilled (probably using @bzimport as the actor).