Why did the rear bumper get lower when I slid the tandems back to the rear of the trailer?

When I was making a delivery at a receiver yesterday, I initially had the tandems slid all the way forward, and the rear bumper of the trailer was too high at the loading dock. The forklift driver at the receiver asked me what would happen to the height of the rear bumper if I slid the tandems backward. I did not know. I told her that I thought it might change the trailer height, but I did not know if sliding the tandems to the rear would make the rear bumper higher or lower. So I slid the tandems to the rear (which pulled the trailer forward off of the loading dock). Then I backed up to the loading dock a second time, and, lo and behold, the rear bumper of the trailer seemed to be about one or two inches lower with the trailer tandems slid to the rear than when the tandems were slid forward. This seems counterintuitive to me. My supposition was that sliding the trailer tandems to the rear would probably make the rear bumper go higher. I imagined that this would work kind of like a see-saw. I thought that the drive wheels and the trailer tandems would both lift the trailer up. The front of the trailer is held up by the drive wheels of the tractor. I thought that if the trailer tandems were slid forward, the only possible effect on the rear bumper of the trailer of the trailer tandems being slid forward would be that the rear bumper would sink down. I thought it might be like a very short see-saw. And I thought that if the trailer tandems were slid backward, there would be no see-saw, and the rear bumper would be higher with the tandems slid backward than if the tandems were slid forward. Instead, counterintuitively, the rear bumper seemed to be about one or two inches lower with the tandems slid to the rear than if the tandems were slid forward. The forklift driver agreed with me that it looked like the rear bumper of the trailer was about one or two inches lower with the tandems slid backward than when the tandems were slid forward.

There was only about 6,000 pounds of freight in the trailer when I was at this receiver, and all of the freight was at the front of the trailer. I think that that might make a difference.

Why did the rear bumper of my trailer get lower when I slid the trailer tandems backward?

 

That's what i would have expected before my experiment yesterday. But when I slid the tandems backward at the loading dock yesterday, both me and the forklift driver thought that the rear bumper got about an inch or two lower.

 

I will do an experiment when I get to my receiver tonight. I will back up to the loading dock with the tandems slid forward, and then I will slide the tandems to the rear and see how that changes the height of the rear bumper.

 

@Hammer166 you could probably provide some useful information on this question.