I think there are 2 things to consider that make a great difference:
The actual use case for notifications. A messaging app like Slack has a pretty compelling use case - you want to get notified of messages you'd miss otherwise. Compare that to the notifications you receive on a tabloid website. Comparing sites with different use cases is going to skew the image you are going to get.
Push vs. pull: does the site ask you to accept notifications with or without any user interaction? Users are more likely to reject permissions that are pushed on them. Unfortunately, the report does not add this kind of data.
I think there are 2 things to consider that make a great difference:
The actual use case for notifications. A messaging app like Slack has a pretty compelling use case - you want to get notified of messages you'd miss otherwise. Compare that to the notifications you receive on a tabloid website. Comparing sites with different use cases is going to skew the image you are going to get.
Push vs. pull: does the site ask you to accept notifications with or without any user interaction? Users are more likely to reject permissions that are pushed on them. Unfortunately, the report does not add this kind of data.
Doesn't slack also not request for notification privileges in the browser until you enable it in slack?
A lot of pages and apps do that, so that would make another interesting property to watch for.