It’s important to understand that when a Nearby Notification is shown (= impression), the Android device doesn’t actually directly hit the URL. Instead, it goes through an intermediary “Physical Web Service”, which validates & ranks the URL, and fetches the web page metadata (title, description, favicon) and returns it to the Android device to use for the notification.
This Physical Web Service probably caches some data (= not all notifications/impressions will actually result in a request to the site), and it might also periodically validate/refresh its cache (= not all requests might actually be a result of a notification/impression).
Because of that, it’s actually very hard to accurately count impressions. You should treat all the impression numbers on an Physical-Web–based dashboard (including our Here & Now) as an attempted approximation.
You can read more about the concept of a Physical Web Service on:
https://github.com/google/physical-web/tree/master/web-service
(Note that this open-source version is not what Google actually uses for Nearby.)