Why is it odd? The two camera bottoms are the same shape and size and the port doors are the same size and in the same position.
If you WANT to spend more money, I believe RRS has a new two-piece 90-degree plate for $185 instead of $140.
It's odd because:
A. Numerous people on LL, FM, and other forums say their RRS 5D3 L-plates fit the 5DS cameras perfectly, including all of the side ports.
B. Someone on FM forums wrote to RRS asking whether their 5D3 plate would fit the new cameras and got the following response:
"Thank you for contacting us about this new camera. Whenever a new version of a camera is released, the first thing we always test is whether or not the previous generation plate will work. That being said, we are extremely picky about the fit and function of our plates on every camera body, because our customers demand a perfect fit and function. There are three specific issues that required a re-design and a new plate for this new camera.
1) new camera does not sit level on the existing plate
2) new camera does not sit flush against the existing plate
3) minor changes to the port side of the camera affect the L-portion fit of the existing plate on the new camera"
C. Kirk is selling the same flat plates and L-plates for the 5D3, 5DS, and 5DSR.
Items A and B above can't both be correct, and item C confuses everything. I've got a RRS 2-piece L-plate for my 1DX. For that it's OK because the camera lives on long tripod-mounted lenses and I've never even installed the "L" part of the plate, but I might want to sometime. On the 5DSR I'd never take the plate apart and I see no point in having a 2-part plate that's always assembled as 1 part, never to be separated, not to mention paying more for it. I think my solution will be to buy a Kirk plate from B&H where I can send it back if it's not a perfect fit. Given the RRS response above I'm not comfortable buying a RRS 5D3 plate for the 5DS, which otherwise would have been my solution for a 1-piece L-plate.
[Edit] The part of the Kirk plate that fits against the bottom of the camera seems to have a shallow rectangular recess around the tripod mounting area and is otherwise considerably different than the RRS plate, which is mostly smooth and flat. It's at least possible that the Kirk plate fits all three cameras equally well, while without that recess the RRS 5D3 plate may not fit the 5DS camera as well.