I did some additional readings, this time using a longwire antenna:
Freq. Day Night
910 -4.8dB 0dB
970 +9.3dB +9.3dB
1080 0dB -3.5dB
On the longwire, I noted that on 1080, the signal strength jumps up and down both day and night. I tried to capture a reading that was close to the maximum. The other stations were stable to within +/-0.2dB. I think that most likely, there is some kind of knife edge diffraction effect being caused by the West Hills. If somebody in Portland could take similar signal measurements, the mystery would be solved.
Here is another signal strength mystery, comparing KUGN (590 kHz) against KXOR (660 kHz), as received from Hillsboro. KXOR has 3 dB more transmitter power, but it should be at a slight disadvantage due to being at a higher dial position. When I measure the signal strengths, using the loop, KXOR is 8.9 dB stronger than KUGN.
KUGN is DAN (directional antenna at night) and Hillsoboro is receiving less then the full 5 kW.
KXOR is only 75 W at night.
On a field strength meter, 9 dB difference is a voltage reading. Assuming day time measurements, 3 dB more power is 6 dB more voltage, not all that far off from 9 dB especially when you consider your location versus the locations of the two stations and all the topography in-between, the electrical length and physical length of the towers involved, etc. etc.