Within a session of Word it will default to the 'last saved' location but if you have only just opened the application, the default save location will be in play.
Since this is a problem for you, perhaps it is worth you having a look at Graham Mayor's addin
Select Folder and Save which will give you an industrial-strength solution to your default save location.
Edit: Hmm, I see you are on a Mac. I'm not aware of whether Graham's addin works on a Mac but it is possible so still worth a look.