Wakeup events from BLE timer (app_timer_set?)

Hi Dialog team,

Sorry that even there are bunch of question relating to the timer usage to wakeup DA14580, I still got strange issue in my application.
In short, my application can't be wakeup correctly if I set a timer (via app_timer_set) with longer timeout (i.e. > 10 s). I use this timer to set a periodically advertising with very low frequency (i.e. to advertise couple event in every 1 min), in which there is no any BLE core activity in between.

When I set timeout to 20s, it then take 67s to be fired, and if I set timeout to 10s, it then take 23s to be fired.
So, timer can be used to wakeup the DA14580, but it's not accurate.
Do I missing something in using such timer? I'm using SDK DA14580_BEACON_3.40.6.

3.2 Wake up events
When in any of the above mentioned sleep modes, DA14580 can be woken up in two ways:
● Synchronously, via the BLE timer which can be programmed to wake up the system in order to serve a BLE event and
● Asynchronously, via the Wakeup Timer and Quadrature Decoder if triggered by an external event (input).