Ember.js with Ember-data. Model object _attributes not persisting to _data on save

I am trying to understand how the ember-data model moves data from the _internalModel._attributes object to the _internalModel._data object.

In my case I am calling item.set('val', foo), where 'val' is a attribute on my item model definition, but when I call item.save() the 'val' attribute is unchanged from the value it had before I called item.set()...

Typically calling .set() then .save() works without fail, even if I'm creating synthetic attributes on the fly. Can anyone explain why it would not persist the value to the _data object to be saved?