Daylight sensor under a roof turns into a Night sensor

Putting a daylight sensor under a roof pretty much inverts the signal. When the time is 18000 ticks it gives a redstone signal with a strength of 9.
at 14500 it starts giving signals.
pic of setup:
https://www.dropbox.com/s/6rza516i9yhmxg4/2013-02-13_23.41.40.png