That's a feature of 64 bit Windows.
In x64 you have two folders with system files: system32 and SysWOW64.
If you put anything in system32, it will be seen by all 64 bit applications. SysWOW64 is there for compatibility reasons - anything put there will be visible by 32 bit (sic!) apps.
So as far as .NET is concerned, the file does not exist. It works for notepad, because notepad.exe is present in both of those directories.
You can test this behaviour. Create a text file in one of the directories, but not the other and check for its existence. Then change your project configuration (right click on solution -> Configuration Manager) between x86 and x64 and enjoy the result.
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