Soo are those the correct dimensions for the psu or what?
The other dimensions for it don't seem realistic .
Hmm, just looked at the Seasonic site myself and I agree, I'm confused myself now. The dimensions you posted ARE exactly what it says on the Seasonic site, but I don't know which site to believe: Seasonic or Newegg.
The 160x150x86mm dimensions are correct, as stated in your initial post your current PSU is 150x140x86. The back opening for the PSU is 150x86mm so it will fit just fine, it will be 20mm longer than your current unit but most cases won't have a problem with that. The 10 inch x 8 inch ish measurement you have is the dimensions of the box they ship it in.
"Most HP Desktop PCs contain ATX power supplies that are 6 inches (15.25 cm) x 5 inches (12.7 cm) x 3.25 inches (8.25 cm). If your PC has a standard ATX power supply, you can replace it with standard ATX power supplies sold by most electronic stores."
If you look carefully, it lists two sets of dimensions, one at the very bottom of the spec section listed as
6.3" x 5.9" x 3.4"
Then in dimensions and weight it lists 10.8" x 8" x 4" which would be the dimensions of the box they need to ship, I don't know why they show that one too on the product order page, they always come packed like russian stacking dolls so they send you a much bigger box, but it does indeed list the correct measurements on the page.
The give away for me was that the only power supplies I have seen near 10x8x4 are benchtop units and certainly not for computers, that would be a very very large PSU for a computer, but 10x8x4 is very similar dimensions to the box my x750 was in, and makes far more sense as the dimensions for a box than for a computer component.