In the case of nitric acid, however, the employment of the aræometer introduces far greater errors than when it is used for sulphuric acid, hydrochloric acid, and in most other cases, as the concentrated nitric acids as a rule always contain nitrogen tetroxide in solution, the quantity being seldom less than 1 per cent., but more generally several per cent., which makes its strength when taken with the aræometer seem greater than it actually is.
Source: wiktionary