Unlike typical food certification, there is nothing exceptionally special about the Star-K certification of appliances. They look at the specifications and functions, and apply their Poskim's understanding of the application of Halacha.
The non-certified one can be shown to any Rabbi competent in that area and he can decide if the mode has any issues you should be concerned about. There is no unobserved process in the manufacture of the refrigerator that would require someone to know something confidential or to have been there during the manufacturing of the fridge in order to say it is fit for this use.
So they may work differently, and there is some chance that the Star-K looked at the GE model and refused to certify it, but the most likely scenario is that GE simply didn't see the marketing value in being certified, and Kitchenaid did.