Distinguishing Brown Ring Test for Nitrates and Nitrites
Yes, you are quite right. It has something to do with prevention of oxidation of nitrites. By controlling the pH, the nitrite present is reduced to nitric oxide ($\ce{NO}$) which reacts with ferrous sulfate to form a brown-colored complex ion, $\ce{FeSO4.NO}$. Nitrates do not react in such way and thus not registered in the test. It uses a special agent to prevent color formation or precipitation of common
interfering ions. Here is the relevant information[1]:


  The high range nitrite test is a modification of the classical brown
  ring test for nitrate using ferrous sulfate. By controlling the sample
  pH, the nitrite present is reduced to nitrous oxide, which reacts with
  the indicator to form a greenish-brown color. Nitrates are not
  registered in the test. [...]
  
  In an acidic medium ferrous sulfate reduces nitrogen in nitrite
  ($\ce{NO2–}$) to form nitrous oxide ($\ce{NO}$). Ferrous ions combine
  with the nitrous oxide to form a brown-colored complex ion, the color
  intensity of which is in direct proportion to the nitrite present in
  the water sample. Color development follows Beer’s Law.
  
  $$\ce{2Fe^2+ + 4H+ + 2NO2‾ → 2Fe^3+ + 2NO + 2H2O}$$
  
  $$\ce{NO + FeSO4 → FeSO4·NO}$$


Reference


https://www.hach.com/asset-get.download.jsa?id=7639984196