Why some extensions have no user count shown?

[Hao Nguyen]

For example:

- https://chromewebstore.google.com/detail/urban-vpn-proxy/eppiocemhmnlbhjplcgkofciiegomcon

- https://chromewebstore.google.com/detail/gmail/pjkljhegncpnkpknbcohdijeoejaedia

- https://chromewebstore.google.com/detail/google-drive/apdfllckaahabafndbhieahigkjlhalf

There are also extensions that seems to show user counts some days, then the user count may disappear for a few days, then back. e.g.,

- https://chromewebstore.google.com/detail/the-qr-code-generator/oijdcdmnjjgnnhgljmhkjlablaejfeeb

- https://chromewebstore.google.com/detail/arduino-create-for-educat/elmgohdonjdampbcgefphnlchgocpaij

What can be causing these behaviors?