I would assume that people lie about their native language in order to attract exchange partners, or students if they are teachers.

There may also be some misunderstanding about the definition of 'native' language.  A number of Filipino teachers on this site claim native English, but on a rigorously upheld criterion, they would not qualify, unless they had learnt English as their mother tongue, probably from their parents or other close relatives, from a very early age.

In much the same way, Indian nationals may be claiming 'native' English, but here again, I suspect this would not be upheld if the definition above is applied.

Does it matter ?  In most respects, no.  These people may well speak and write excellent English, possibly better than many genuine native speakers.  They are well-qualified to help others to learn the language.

However, when it comes to idiomatic and colloquial usage, they will be at a disadvantage, as this knowledge is accrued by 'growing up' in the native environment of the language, and cannot easily be acquired indirectly.  It is for this reason that people should be scrupulously honest in declaring their language status on this site, so that prospective partners or students may decide whether they would be appropriate as partners or teachers.

Yep, I'm a native in double Dutch.

Oh damn, you got me !!!

what am I supposed to do now ?

Given he teaches German I would imagine he is a German native speaker. Maybe he has one other native language and is bilingual but three? Hmm... For a start, his English has some errors on his profile, the first sentence in his about me for example. 

On another note, I have only just noticed that green signifies native language, and pink signifies a learned language on the profile. Bit slow on the uptake there I guess. Now my previous question about why you asked this question seems a bit obsolete Aegis...

I like direct up front questions. No bs.

People really need to stop lying about native languages.

To say that you are a native speaker at something just because you are good at it is, well, stupid.