The real explanation is much simpler (and less interesting) than some offered above. 

You learn *dactylonomy*—finger-counting—when you're an infant. Little Archie Hickox learned it from his Irish parents, who passed the art on, unchanged, from their own infancy, *und so weiter, und so weiter*.

As soon as you can do basic arithmetic, finger-counting becomes pretty irrelevant to your life, unless you order a lot of beers or work with a lot of trauma patients. You could theoretically go your whole adult life without ever counting on your fingers...

...until you have kids of your own. Then you enact the same, indelibly-rehearsed motor actions your parents instilled in you.

Archibald Hickox grew to be a fine fellow and a better-than-average linguist. His love of German caused him to spend ***no end of time among no shortage of native speakers***. 

He could have spent a thousand years in the company of his echt German friends; it would have helped him shake off the Irish accent, to be sure, but it probably wouldn't have had any influence on his dactylonomic habits. After all:

1. he was hanging out with them <i>to improve his German</i>
2. he <i>already knew how to count on his fingers</i> and, as a grown man, it wouldn't have occurred to him that he had to re-learn a skill associated with babies
3. anyway, finger-counting cultures are *mutually intelligible*. Note that the gesture was perfectly effective: just before Hellstrom leveled his pistol at Hickox' *Eier*, the bartender delivered exactly 3 glasses to the table. 

The ScheiBe hit the fan because Hickox violated what linguists call an *ethonomathematical shibboleth.* 

If Hickox had learned his German in the context of *training for espionage*. Any decent instructor would have drilled Hickox on the life-or-death importance of dactylonomic and other shibboleths, particularly because these are somewhat quicker and easier to master than  a clean foreign accent. 

But you have to know that you HAVE to master them. And we can't expect someone who's not a professional spy to have thought of these things.