Patrick,
I may be all wet on this (pun intended), but .............
I think we're trying to simplify a complex process within a complex environment, a bunker.
But, simplistically speaking, using the chart below (for some unspecified kind of sand), the right end represents how much water is in an air dried column of sand. As you move left on the curve you come to the field capacity of the sand column - that is the percentage of the column that could be water (9%) before gravitation starts to drain the water out of the profile. The left end of the scale is an indication of what the percentage of water is in the column at saturation (25%).
So, if you have a bunker with 2" of sand, adding 0.18" of water will make the bunker wet, but it won't be draining. As you add water, the sand becomes more saturated and starts to drain, up to 0.5" of water when the sand is totally saturated. After that, I guess puddles form unless the drainage rate is faster than the rate at which water is added.
If you have a bunker with 10" of sand, you would have to add 0.9" of water to reach the field capacity of the column and get the same level of wetness as only requires 0.18" if the sand was only 2" deep. Similarly, to reach saturation you would need to add 2.5" of water to saturate 10" of sand vs adding only 0.5" of water to saturate 2" of sand.
How fast the bunker dries out probably depends on lots of factors including weather, drainage structure, and sand composition. In dry hot conditions water will evaporate rapidly during the day. Those in the sun faster than those in the shade. If the profile is approaching saturation, some will drain off. The water content of the profile will be constantly changing.
What is the ideal water content for a bunker where the sand is not too dry, but not too saturated? I don't know. I'd guess it'd depend on the person/player assessing the playability of the sand. Some probably prefer it drier and fluffier and others, probably wetter and firmer. I doubt that there are very many courses, if any, that try to manage the bunker wetness/firmness in the same fashion they try to manage watering the rest of the course.
So, if your sample course waters daily and covers the bunkers, and if they apply 0.5" of water a day, and if the bunkers have only 2" of sand in them, then they are likely to be saturated most of the time and not likely much fun to play out of. And the continued wetness probably leads to moss and other organic growth.
On the other hand, if they have 10" of sand in the bunkers, and they applied 0.5" of the water the profile would probably be comfortably dry to play out of, as you only be at about half the field capacity of the 10" of sand.
Do courses usually apply 0.5" of water a day to greens? Seems like a lot to me, but I have no idea.
Of course, the real world would be a lot more complicated than this with ever changing weather and different drain approaches and different shapes and contours of bunkers and different depths of sand.
By the way, the information Dane provided in the orange post, and that you questioned, was from the USGA. Follow the link to see the article. You'll note that the article was written by Keith Happ and his phone number and email address are at the bottom of the article. He invites questions and the article is only 2 months old, so why don't you pursue your questions with him. I'm sure he's much more knowledgeable than I am.
http://www.usga.org/course_care/regional_updates/regional_reports/northcentral/Hazard-Ahead-%E2%80%93-Sand-Depth-Matters---September-2013%282%29/