Dry Electrode with High Impedance

I use the TDE­200 Disposable /Reusable Dry EEG Electrode, without any gel, along with Ganglion board and Ultracortex cap. I observed that:

1) when I short the electrodes with the ground, the impedance could be as low as 0.5 K ohm.

2) when I contact the electrode and ground on my hand (different fingers), the impedance is as high as 700 K ohm.

3) when I wet my figures and repeat 2), the impedance drops to 5~10 K ohm.

These electrodes were purchased around 1 year ago. Unfortunately I didn't measure the impedance at that time.

I was wondering if anyone can help with the following questions:

1) why is the measured skin impedance so high?

2) could this due to "oxidate" effects for this type of electrodes?

Thanks,

Li