False ceiling... Why?

I was cutting some holes in my ceiling today to install recess lighting. During prep, I noticed that the beams were running perpendicular to the direction I expected them to. I cut my first hole, and began fishing over to an existing fixture... Lo and behold, I could fish perpendicular to the beams the entire 5 feet, no drilling!

I began investigating, and found that my 8' ceiling is actually dropped down 6" from the original ceiling, attached to 2x6's that based on their color are about 20 years old. The 2x6s that I can see are not flush with the original ceiling, instead dropping down about a half inch, providing the gap that is making my life easy for fishing. I haven't yet found any point where they are actually attached, but I've only looked through a few holes, out about 9" in all directions.

So, the question is, why is the ceiling dropped like this? I mean, if they just wanted to put rock over old bad plaster (though the plaster where I can see is in very good shape), why fir down so far? Why take 6" instead of just 1"?

Adding to the mystery, the ceiling in the dining room (off the LR) is 8'3", and the kitchen (off both the DR and LR) is 7'9". I'd noticed the kitchen height before, but I actually figured they'd dropped the whole ceiling to work around a beam there... Hmmm, that gives me an idea for a reason... Maybe they took the entryway and LR and took out a load-bearing wall, put in a beam, dropped the whole ceiling to hide it? I think I'd rather have the 6" everywhere else. I'll have to find a good way to probe for that.

Any other ideas for a reason?