Timer invoke its block so quickly when enter background and enter foreground after a while

Question

let _ = Timer.scheduledTimer(withTimeInterval: 4.0, repeats: true) { (timer) in
    //do something
   }

I create a timer and invoke its block every 4 second. Then I make application to enter background and enter foreground after a while. But it could invoke the block quickly even less than 1 second sometimes.