Difficulties with 4k60 support on my Late 2013 27" iMac
DisplayPort v1.2 -- vs 1.0 and 1.1 -- newly supported 60Hz UHD (4k), where the older specification had only enough speed to support 30Hz.  [Search "specifications displayport".  (On the Wikipedia page, you have to look at two separate tables.)]

I would infer that DisplayPort port on the older machine was v1.0 or v1.1, with the newer machine's being v1.2 (or higher).  (Apparently your cable is v1.2 (or higher).)

Since the two machines are running the same version of the OS, I would guess that the difference in the presentation of the resolution is a function of the DisplayPort version... although I would have thought that it was the OS itself.  ["Larger text" means (non-native) larger dots on the same screen (and fewer of them), and "More space" means, in the last case, full native resolution.]

As for the Apple specification page... one might wonder if there was an improvement in the hardware capability enabled by the software (i.e. the OS)... but I would guess that it actually was a mistake (surprisingly enough); the NVidia specifications for all the pertinent graphics cards say 3840x2160.  [I note that Apple has been known to cripple its hardware -- as in, such that you can get more than advertised, out of it -- on occasion [ref. incl. my own machine], but I can/do not see that here.]

BTW, my experience with my dual-panel display (which I think is "MST") is that you can catch it out sometimes, with one side lagging behind the other briefly.