Stretching frequency of nitro and nitrito
First of all, in the webpage, I think there is some shuffling between Nitro and Nitrito first in the headline. But, according to the contents, Nitro has closer stretching frequency, but Nitrito has distinguished stretching frequency.Let's take that one as the correct data.

The stretching frequency of nitro ($\ce{&lt;-NO_2}$ )corresponds to the stretching of the bonds between $\ce{N}$ and $\ce{O}$, which are in between double and single bond, due to resonance, and the bond order between these $\ce{N}$ and $\ce{O}$ becomes almost equivalent in both the two  bonds.      

Thus the stretching frequency of $\ce{-NO_2 }$ corresponds to partial double bond character between N and O and almost equivalence of two bonds.  On, the other hand, the two streching frequency of nitrito ($\ce{&lt;-O^ -N=O)}$ corresponds to one $\ce{N -O}$ pure single bond and  one $\ce{N=O}$ pure double bond as there is no delocalisation in this case. So, due to that pure double bond stretching frequency of $\ce{N=O}$, one streching frequency of nitrito is higher than Nitro, which is consisting of only partial double bonds.