We provably aren't seeing that, as there are characters (Kor, L'rell) who have appeared in more than one variant of the Klingon makeup. The implication is obviously that these are different interpretations of a single species' appearance.
"Empire" generally implies "imperialism" which generally implies a racial hierarchy. Or at least that's one argument.
I woulda liked "four nostrils, but otherwise the TNG design" to become the standard.
Oh, what if it's a World War 3-era combat drug, like the stuff Q alludes to soldiers being addicted to in Encounter at Farpoint?
I was surprised recently to realize that it might actually be "Time's Arrow"-- the more so because I'm not as much of a TNG person as I always thought I was. But Time's Arrow is a fun romp through a nifty and underserved setting, and the ending with Mark Twain is I think one of the most important sequences in TNG.
Also there had better be a good explanation in future episodes as to why the hell Dr. M’benga keeps a couple of vials of punching serum handy at all times.
He was a frontline doctor during a conflict with an enemy that considers medics viable targets.
Structural differences in their larynx, tongue, or lips seem likely.
I must say I'm already a big fan of Pelia.
I certainly would as well. (Actually I'm glad you mention Romulans as they provide another data point regarding this matter. The same individuals-- the Bird of Prey Commander and Sub-Commander-- at the same time-- the shared events of "Balance of Terror" and "A Quality of Mercy"-- show different variations of the Romulan makeup.)