US Forces have adopted the Delta Point Pro, after extensive testing. Sig therefore made the M17 / M18 compatible with the Delta Point Pro. Sig did not want to loose out on potential sales so they made their newer Romeo 1 Pro the same mounting pattern as the Delta Point Pro. They also made some durability improvements to the Romeo 1 Pro. This was a very smart business decision but Sig. It is always a good thing to share a mounting pattern with another very popular Red Dot sight and allows for many more options for the consumer. The Sig Romeos also mount quite low and have the notch cut out in the back to allow the emergency sight picture with the existing front factory sight in case the dot fails. Again, very smart as there is no need for big ugly suppressor height sights.
Rich