Observer observes daylight sensor twice

If a daylight sensor is placed in front of an observer it will output for the block placement and then another one randomly. Sometimes it is instant, other times is takes a second, and there are also times it doesn't do it at all. Just to make sure I set daylightcycle to false and it still does it.

Edit: the second pulse occurs from the daylight sensor updating on the 20th tick. However there is a chance that placing it at the right moment will cause the observer to give a single output as both the block placement and the update happen at the same time.