Rigidty of basket affecting Fs ?

I thought I would share this with you. Whether what I am seeing is real is another question......

I have been measuring the T/S parameters for some Supravox 400's I have acquired. If I have the driver cone up on the bench, I get fs=26hz. If I attach the driver to a flat baffle of significantly stiff/weight, fs =23hz. I have repeated this a couple of times, changing the distance of the back of the driver from adjacent surfaces, and the baffle in different orientations, same results. I don't think it's additional air loading off the baffle, change is to big. All I can think that the additional mass loading on the basket, is affecting the the resonance of the cone assembly somehow. This goes against my understanding of vibration, it's part of my day job, understanding vibration of complex assemblies that fly!!!
Can someone explain this for me, to save my sanity.