I have suddenly had a very similar problem, and I strongly suspect that a recent software update for the P800 from Epson may be responsible. Here is my situation:
I do all my printing from Lightroom 6.1, and I have a substantial set of printing presets for common sizes of paper that I use (most often 8.5x11" for test prints, and 13x19" for display prints.) One of my commonest prints sizes is to fill a 10x15" area centered on 13x19" paper. Thus, an image that is actually 10x15" will have borders of 2" on the short sides and 1.5" on the long sides. Prints of other sizes expand to fit either the 10" or 15" of the image area. This preset has worked for me perfectly for as long as LR 6.1 has been out, and I have made hundreds of prints using it.
I printed a couple of prints using this preset just a few days ago, after not printing since February. The P800 did get a new software update before I started printing. The Print Preview showed each print centered perfectly within the 10x15" region, with equal margins on the sides and equal margins top and bottom. However, the actual prints came out with the short side margins of 1" and 2", an effect very much like that of the original poster.
After checking all my settings a dozen times and updating my preset, I made test prints on some 13x19" paper that I don't use any more. All prints were off center in the same way. Curiously, I tried a couple of prints on my standard 8.5x11" preset, and that worked perfectly, with correct margins, as it has hundreds of times in the past. The problem seems to be specific to 13x19" paper.
Next, I tried a workaround: I created a new preset specifying margins of 0.5" on the side that printed with a 2" margin and 2" on the side that printed a half-inch margin. I tried this preset on two sample images. Each time the print preview suggested that the margins were now wrong in the reverse direction from prints using the previous preset, but the actual prints came out perfectly centered.
So this new preset seems to compensate correctly for the error that creeps in somewhere between computer and printer, so I will use this new preset for 13x19" paper until Epson (or whatever malicious mischief-maker) restores the pipeline.
I did also try one sample print using PhtoShop, and the same unequal margins appeared, so I strongly suspect an error in Epson'd latest update.
Sigh!
-Eric M.