I am thinking marketing, because I see no differences in the Ridleys, and here are my specifics

Start with 58cm Roubaix vs. Tarmac. Both have effTT of 582 and STA of 73 deg, but the Roubaix has taller head tube (225 vs 205) and a slacker HTA (72.5 vs 73.5). So I can see specific geometry differences that say Roubaix is targeted toward a slightly less agressive, more relaxed steering (I lack fork rake but assume them to be consistent), hence this is Spec's "endurance" geo.

With Ridley, Damocles vs Excalibur (which is the one I am ogling), the numbers are identical for all sizes, specifically the large being effTT of 585, HT of 205, STA 72.5, HTA 73.5. They are almost an exact match to the Tarmac in that size, and are identical to each other. They even list identical stack and reach.

I know that spacers and stem can more than make a race bike into an endurance, I just found it very odd that all of the Ridley models share that exact same geometry, but advertise themselves as different.

Largely just mental exercise anyway, as the most likely next candidate for me is a Surly Pacer.