I am not a chemist, so my explanation may mean nothing, but, this is what I understand:
Sodium Nitrite becomes nitrate when it is exposed to oxygen. As such, I assume what happens is the oxygen combines with the nitrite. Now SN causes the ctb by causing hypoxia, this is, your bloodstream fails in carrying oxygen to your brain rendering you unconsious. But if Nitrate is nitrite but with oxygen on it, then it shouldn't be able to cause that hypoxia at all.
I know nothing of chemistry nor medicine, so do not take my words for granted or even remotely correct.