anchorelectricallight
I came home to a light fixture hanging off the wall today. The light was never attached to the electrical box, instead it was installed with drywall anchors to the edge of the drywall against the box. The drywall was covered with caulk and big pieces of it (caulk) came out with the anchors.
My question is why would someone do this? Is there a reason why you would not install the light to the box? I would think the drywall anchors would be more work than just screwing it into the box. 
I suspect the white wire with the stripe is actually the switched power from the switch. Putting together what you have said, I think your setup probably looks something like this:
Where the green wire is the ground and the red wire is your wire with the stripe.
If this doesn't match up with what you are seeing, then let me know what looks wrong and I will try to figure out how it is setup.
Before you start, make sure the power to the fixture is off (not just at the switch) and, if possible, confirm with a non-contact tester (a few bucks at a hardware store).
D are the screws that need to be put through B, which is the bracket that will be attached to the ceiling box to hold up the fixture.
To see which holes, align the bracket with the holes in the fixture base. Thread screws D through the back of the bracket so the threaded ends stick out front. Which side of the bracket is the back depends on the ceiling box and whether you need the offset on the bracket sticking down or up. (It often doesn't matter.)
C are the screws that hold the bracket B to the ceiling box. Once the bracket is installed, you can connect the wires on the fixture to the wires in the ceiling box (including the ground wire to the green screw on the bracket).
Place the fixture against the ceiling with the threaded ends of D sticking through the holes. Attach with the nuts E.
The threaded tube on A is screwed into the center of the fixture, the nut is then tightened down the tube to the fixture. The remaining parts are removed. After bulbs are installed, the globe is put on, followed by the soft washer, the hard washer and the next nut. Don't overtighten or you can crack the globe. Then the decorative cap is put on and then the ball finial.
Best Answer
I am with everyone's opinion that we can't explain why someone install something wrong. Other than he did not practice standard installation practices, and that probably means that whomever installed the fixture was not an experienced electrician.
What I can tell you is that all of the necessary parts are there in your pictures. First your box looks ok. You can see a lug attaching to the box at the 1/2" knockout in the back indicating a brace. also you have the mounting plate for the fixture. That's the round thing with all of the slots and holes in it. You will find that one set of those holes and slots will line up with the screws in the ears of the box. Then another sets of holes will line up with your fixture.
What I don't see is the ground coming from the box. I hope whoever installed it didn't just cut it off, because that need to be connected up also.
If an electrician or an electrical contractor you need to contact them and have them make it right and if they don't file a complaint the your AHJ.
Good luck.