You could easily argue that the Ambassador has two major variants: the one shown as the Enterprise-C, and the one that was otherwise seen with different bussard collectors, saucer placement, impulse engine arrangement, and extra aft shuttle or cargo bay. Not necessarily a radical change as the Connie or glued-on-bits mania like the Excelsior, but a variant nonetheless.
My take on the two distinct "variants" is that they're analogous to the Excelsior vs. the Enterprise-B: A few different odds-and-ends, but not enough outward differences to warrant the ships not being the same class.
Of course you could argue the exact opposite with the Miranda vs. the Soyuz, but that's different. The modifications made to the Reliant model to turn it into the Bozeman were an issue of necessity, not design.