What does でしょなんでだ mean?


[![enter image description
here](https://i.stack.imgur.com/ZhiVN.png)](https://i.stack.imgur.com/ZhiVN.png)

What is the meaning of the sentence on the left exactly? She seems to be given
the reason the forest is called like this is because of the fungi?




She's confused why it's called "Poison Wood" when the forest isn't poisonous.

The last two phrases

  * (a) フシギでしょ 
  * (b) なんでだ

should be read as two separate sentences.

  * (a) "How strange."
  * (b) "Why?" (perhaps more rhetorically than as a real question, as @naruto suggests)

