Why do I get different browser resolutions for a mobile device?

It depends on the orientation of the device - landscape or portrait.

The browser resolution is a session-related information. This means that there can only ever be one resolution per visit, which is sent by the pixel.

If you change the resolution within the visit (e. g. Landscape to Portrait), the last used resolution is shown.

If you perform several visits with the same device, there may still be several recorded resolutions due to the reason mentioned above.