Hi,
I think it´s rather clear what happens here.
5.5mm and 2.1mm are the "nominal" values. To give the connectors a common name. Indeed meant that it´s more easy for users to chose mating partners.
And for them to fit you need to use "tolerances". That´s why the production drawing has different values.
If you don´t use tolerances and you produce a 5.5mm hole and produce a 5.5mm plug you have a 50% chance that it will not fit.
Thus you need to produce the hole "a little wider" and the plug "a bit smaller" for them to fit 100%.
It´s a quite common pratice in mechanics.
Added, about the length.:
here you may use plug that is longer that the hole is deep. Thus you uensure proper operation even if the jack is mounted behind a 1.5mm thick wall of an enclosure.
Klaus