I am lying here watching a film and have to hand a Samsung tablet and Redmi Note 4 phone both connected to the internetweb via WiFi both give an audible sound when a notification is received for an email or Facebook. My question us why do both devices receive the notifications at different times sometimes there are minutes between the devices receiving the same notification? Over to you BT.
Each device periodically asks the email or Facebook server if there are any new messages. They're just asking the servers at different times, that's all.
As @bawjaws said, each device [and indeed app] performs background updates at different intervals, also some devices stop requesting updates after a set period of inactivity in an effort to conserve battery.