The movie also has Uhura taking over communications because the senior communications officer can't tell the difference between Vulcan and Romulan.
So there's three dialects of a language very close to Romulan. It would be like an American not being able to understand someone from the UK and being replaced by someone who can understand English with Scottish, Irish and Australian accents.
Maybe it's like Serbo-Croatian
, where there can be a common literary standard, but there can also be separate literary standards, and the ethnic identities of speakers don't necessarily map onto dialect areas.
Or, who knows? Maybe the Romulans of Artaleirh and Archernar Prime just speak with really thick brogues.