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

Please provide a explaination for the same (the title question).

• Well, maybe this is because they are not single bonds? – Ivan Neretin Jan 14 '19 at 8:17
• Please cite the sources for the quotes and images you use, even (or especially) it they are from of own network. – Martin - マーチン Apr 14 '19 at 19:32
• "Which is definitely untrue and incorrect" Why do you think so? – Karl Apr 14 '19 at 19:56
• @Karl “That is thoroughly incorrect. chemistry.stackexchange.com/questions/51168/… chemistry.stackexchange.com/questions/29101/… – Mithoron” – Chemist Apr 14 '19 at 21:07
• Please explain this (in one or two scentences) in your question. It's not obvious. – Karl Apr 14 '19 at 21:12