Germans in London

A group of Germans went on holiday to England and the first afternoon they went to a typical London pub.
One of the Germans had studied English for years and really put an effort to speak free from the typical German accent.
The waiter came to their table and he started ordering the drinks,
of course in perfect London English:
"We would like to have
2 coffees,
1 red wine,
4 Guinness
and 2 Martinis."
The waiter went back to the bar and noticed he had two different types of Martini.
So he went back to the Germans and very politely asked:
"Ahum, excuse me, is the Martini ........ dry?"

The German turns red, gets up from his chair and shouts at the waiter:
"NEIN ..... ZWEI!!!"

(Note for blondes: dry sounds like DREI, which means THREE)

I don't get it.

Dry (Drei) is the number 3 in German. Zwei is the number 2.