the better question is why didn't that show up in medical scans. could it be all this time he's avoided or faked medical scans?
Vulcans and Romulans are physiologically very similar. 3Human-1Vulcan hybrids and 3Human-1Romulan hybrids presumably are four times more similar than Vulcans and Romulans. The difference is probably subtle enough that you wonít find it unless youíre looking for it.
They probably donít look for it routinely, because there is no law against people of 25% Romulan blood serving in Starfleet. (Saavik was 50% Romulan and served openly in Starfleet.) Tarses lied on his application in an attempt to avoid the prejudice and suspicion with which Romulans are typically regarded, not because the truth would have kept him out of Starfleet, but because the truth would have made his life in Starfleet a lot more difficult.