Check the timestamps in the directory and you'll see any hours, not only night timestamps. What is updated every 8 hours are the builds made during the latest 8 hours. That way of functionning is legit, I'm not complaining. A daily update would be fine, so an 8 hours basis is almost luxurious 
https://downloads.openwrt.org/snapshots/targets/
For example this has been build an hour ago, and is not already available in the image selector. My guess is that it will be at 14:00 UTC.
https://downloads.openwrt.org/snapshots/targets/bmips/bcm6358/
I'm motivated by scientific curiosity to understand a very convinient tool I'm using. The very same that had lead everyone here to try OpenWrt instead of the stock firmware 