Dude, you really are making Americans look as arrogant as the world would like them to be.
Seriously.
Wikipedia first, LFS Forum second.
As a general rule, many cars from the same manufacturer share the same platform, engines, etc. It brings R&D as well as production cost down while giving the customer a variety of choices.
As far as VW, generally the following cars share the same platform:
Golf, Jetta, Beetle as well as all the variations of those (convertible, wagon, etc)
There is nothing "tuned" about the Scirroco.
It was just developed around the same platform.