Although this may be generally true, looking at both segments is helpful. NWA may offer more seats for BKK-DTW than BKK-NRT because the connecting flight to DTW brings in more $. I look at both to get a better picture. However, when you look at them individually, you can usually identify which one of the flights is the limiting one.

BKK-NRT does seem heavily booked this month. However, I have never received an op-up or bump departing from BKK -- has anyone else? (from NRT to BKK is a different story.) Maybe its because I don't get to the BKK airport until 5am for the 6am flight, and so they've already bumped people or identified volunteers before I arrive?