Elf spawn

OK, I have been taking notes on the number of elves generated each time i login and it is indeed 3 every half an hour except:

If I login every half and hour to collect them, the game seems to add the "collection time" to respawn time, i.e. the 30 minutes timer starts only after i collect the last one from from the current respawn
Sometimes, on average maybe once a day. I seem to be randomly getting an "extra respawn" - looks like some glitch? Also sometimes more elves than expected are generated during the night.
On 27/12 (definitely after the Xmas bonus period) I had a few hours with higher and irregular respawn rate.

Apparently there is some bug in the game that under some conditions can produce additional elves. Wish I knew how to exploit it :)