While not canon, the book "My Enemy My Ally" by Diane Duane has the following quote from McCoy, which gives a physiological reason.

The quote is discussing a plot to invade a Romulan station with the aid of Romulans (matter of honor), and having the Rihannsu act as Vulcans with each boarding group:

"They won't think that for long, Spock," McCoy said. "Remember, Rihannsu and Vulcan culture have been diversing for thousands of years...and most of the subconscious cues buried in their respective kinesics, their 'body language,' will also no be very different. A Rihannsu would know you weren't one, if he looked long enough, not from any physical divergence- but just from a wrong 'feel.'"

So basically, while a Vulcan may outwardly look like a Romulan, there are ways of walking, acting, talking that are endemic to any culture, and if a person does not fit that, they give off a wrong vibe to people of the culture.

While not canon, the book "My Enemy My Ally" by Diane Duane has the following quote from McCoy, which gives a physiological reason.

The quote is discussing a plot to invade a Romulan station with the aid of Romulans (matter of honor), and having the Rihannsu act as Vulcans with each boarding group:

"They won't think that for long, Spock," McCoy said. "Remember, Rihannsu and Vulcan culture have been diversing for thousands of years...and most of the subconscious cues buried in their respective kinesics, their 'body language,' will also now be very different. A Rihannsu would know you weren't one, if he looked long enough, not from any physical divergence- but just from a wrong 'feel.'"

So basically, while a Vulcan may outwardly look like a Romulan, there are ways of walking, acting, talking that are endemic to any culture, and if a person does not fit that, they give off a wrong vibe to people of the culture.