esp_wifi_sta_get_rssi clarification

[ray1050]

I would like to know how esp_wifi_sta_get_rssi() function gets RSSI? And why it is not updating on beacon interval?
Doc string says "rssi: store the rssi info received from last beacon." Router sends a beacon frame on 102.5ms intervals, so I am expecting RSSI to roughly update at 102.5ms.
In my experiment, I am calling esp_wifi_sta_get_rssi() on 200ms interval but RSSI is only updating on an approximately 1 second interval.

Thank you.