Question setting the duration of sleep mode

[EmcSquared]

Hi,
Happy New Year everyone.

I have an issue/questions with duration a node goes to 'sleep' mode using RF24Network::sleepNode:

• whatever I do it seems that the total sleep duration is not the product of 'number of cycles' and cycle period (as set by 'setup_watchdog'). Here is an example:
  I used this line to set the cycle:
  network.setup_watchdog(wdt_8s);
  and these to enter the module in sleep mode:
  radio.stopListening(); radio.powerDown(); network.sleepNode(212,0); radio.powerUp();
  Now the total sleep time should be 212*8 = 1696 seconds, but it is not; It is 1918 seconds, every time!
  I am using Arduino Uno R3, and what I am trying to do is to send a sensor reading every 30 mins.
  Am I missing something here?

Thanks for your help