Reason for shorter bond between sulphur and oxygen? I recently came across this sentence in my textbook:


  the bonds between sulphur and oxygen in oxides of sulphur ($\ce{SO2}$ and $\ce{SO3}$) are much shorter than might be expected for a single bond.


 



I feel it could be due to partial double bond character due to resonance.

And the same textbook gives another explanation:


  In these molecules, in addition to normal π bond, a π bond is also
  formed by the sidewise overlap of a filled 2p orbital of oxygen with a
  vacant 3d orbital on the sulphur). This is called pπ - dπ bond and
  results in bringing the two atoms closer and thus accounts for shorter
  bond length of $\ce{S-O}$ bond.


The reason provided in the textbook (as mentioned above) is definitely untrue and incorrect

Because of reasons mentioned in these posts 

Why is the bond order in the SO₃ molecule 1.33 and not 2?
.
Hybridization of sulfur in sulfur dioxide 

Please provide a explaination for the same (the title question).
If you would consider it more carefully, you would see that the bond order in SO2 and SO3 are, respectively, 1.5 and 4/3. This means that they are shorter than a single bond of sulphur and oxygen.