Things that make you say "DUH" - what am I missing?

Please explain this huge plot hole "Duh" to me.

When I was a little girl I had a code breaking book and the first rule was you always started by looking for a highly repeated three letter word to start. In English, it's always "and" or "the." "The" is the best bcs it gives you more common letters then "and." Obviously, they are words constantly used in English.

Second trick - decode by eliminating consistent patterns. We used to do this is Girl Scouts puzzle/code games.

So, it seems ludicrous to me that they would not have 1) started with the consistent daily weather announcement that would have had very few variables and 2) that "Heil Hitler" would not have been the first words they tried to decode. As far as I know, every communique in the Third Reich had that phrase in it.

This has been bugging me for a week. For the life of me, I can't understand what I am missing! What am I not understanding about their decryption process?

Absolutely amazing, complex and heartbreaking movie. A new Top 10 for me.