It's also possible that they can't track new installs either.
FAQ:
How is Unity collecting the number of installs?
We leverage our own proprietary data model and will provide estimates of the number of times the runtime is distributed for a given project – this estimate will cover an invoice for all platforms.
Which is some kind of weird nebulous BS.
They're not saying their engine phones home and/or collects data from end-user devices. With the associated data protection nightmares.
Oof. This is corporate lingo for "we'll pull a number out of our ass and charge the dev accordingly". "Proprietary data model" makes it clear they intend to remain conveniently (for them) opaque about it.