Licensing a work using a GPL-ed library under the LGPL

[Shriramana Sharma <samjnaa@gmail.com>]

Hello.

The matrix under

http://www.fsf.org/licensing/licenses/gpl-faq.html#AllCompatibility

says that a work using a GPL-ed library can be licensed under the LGPL 
"if you convert to GPL". For example the KDE libraries which use the 
GPL-ed Qt libraries are themselves licensed under the LGPL.

But I don't understand what is the meaning of licensing something under 
the LGPL if it is effectively under the GPL anyway because it uses a 
GPL-ed library. So what is the point in licensing it under the LGPL?

Could it be that parts of such a work which do not use a GPL-ed library 
can be used under the LGPL? Is there such a legal distinction between 
"parts that use the GPL-ed library" and "parts that don't"?

A related question is about the people who have contributed code to Qt 
under BSD/MIT-style licenses (see 
http://doc.trolltech.com/4.3/licenses.html). They have definitely used 
Qt's own classes in their own code. I also don't think that all of them 
would have got a commercial Qt license which gives them the right to 
license their code in whatever way they wish.

In that case, how can their code which uses GPL-ed Qt be licensed under 
anything other than the GPL? Is it that the copyright owner of Qt has 
granted them special permission to license their code under those free 
non-copyleft licenses? If so, isn't it necessary for legal propriety to 
mention that in that page?

Thanks.

Shriramana Sharma.