I believe this is because when you don't give a specific name, it's matching the (normally-running) EvernoteHelper app instead.
I think that as long as you have Evernote itself running at compile time, the correct reference will end up in the script, so a compiled script should fire up Evernote instead of the EvernoteHelper when it's run.
You might find that using the bundle identifier "com.evernote.Evernote" instead of the hardcoded path to the app works, too:
tell application id "com.evernote.Evernote"
(Note the "id" part.)