Single Wall Top vs Double?

[Jimma]

This might sound like a dumb question, but follow with me....

I have the top on my Jimmy, it is what I believe to be a single wall top?
-it has interior lights
-mounting points are visable from inside the truck
-take about three guys to lift and move

Now I have another top which I think is a double wall....
-the mounts for the bed sides are inside the top
-no interior lights

now here is where I am confused.....the second top is lighter then the one I thought was a single wall??? I could easily pick it up with me and one other person.....

Do I have the tops mixed up or what?

If not I thought the single walls were more desirable because they were lighter?