Theoretical comparison of dipole moments of N,N-dimethyl-4-nitroaniline and N,N,2,3,5,6-hexamethyl-4-nitroaniline These two compounds are given in my workbook, and I am supposed to compare their net dipole:



The solution given in the workbook is that:


Molecule A (N,N-dimethyl-4-nitroaniline) has a "net shifting of electrons" in the downward direction - due to $+M$ effect of $\ce{NMe2}$ and $-M$ effect of $\ce{-NO2}$;  
Molecule B (N,N,2,3,5,6-hexamethyl-4-nitroaniline) - due to steric hindrance - does not undergo resonance to an appreciable extent, hence, there is negligible "net shifting of electrons" in any direction. The lone pair of $\ce{NMe2}$ almost remains in its place.


Hence, molecule A has a higher dipole than molecule B. I am content with this explanation.

However, my question is:


  Is this explanation actually correct, or is it just a theoretical mask to explain an experimental observation (i.e. they just made up a random theory on their own to explain an observation)?


Are such types of explanations common, universally accepted, etc?
The workbook's explanation could have been more specific. If you build a 3D model, you may see that the orbital interaction between the lone pair on the $\ce{NMe_2}$ and the $\pi$-system on the ring is somewhat hindered by bad overlap because the steric interaction of the methyl groups (on the ring and the $\ce{N}$) prevents perfect alignment. Same for the nitro substituent. A higher interaction would lead to more charge shift.