Knowing a language perfectly with respect to grammar and pronunciation is no guarantee to blend in with the natives unnoticed. A language is not only pure means of communication, but also expresses the speaker's cultural background and mindset.
According to from which country your dialog partner comes from, he does not only want to get his message through, but also wants to communicate his personality. English as the world´s communication tool No. 1 soemtimes reminds me of the Babel Fish, responsible for many intercultural Tsunamis, its users not being aware of the fact that knowing the language alone, does often not suffice.
The Germans famous for beer and cars, are also notorious for their direct way of communication.
While a Spaniard would require a pretext of half an hour, why you would not want to meet up for a beer, a German would just say “NO” and considering “I don´t have time“ enough reason for this harsh turn down.
In order no to be offended, keep in mind that the Germans love efficiency. While for a Brit it is not enough just to get his message through and fritters half the day away in small talk and unsubstantial banter, your standard issue Fritz loves to gets his things done and does not feel the need not be forcefully witty and to apologize for rejection. Neither does he take it personally when the tables are turned on him in this respect.
What can freshen up an international meeting more than a Germans direct negativity and straightforward questions that make all participants squirm with embarrasment? It´s like a shock therapy and make even the most entrechend opponents losen up.