ESD Anti static mat Grounding and AC volts
Your body is picking up AC voltage from some source.  This is a common phenomenon. 

The amount of current that is available is dependent on many factors.  From the measurements that you give, I would approximate that current to be about 8V / (1 MegOhm + 1 MegOhm).  Note that this doesn't include skin resistance but that is often quite low after you have been wearing a wrist strap for a while (sweat).

Note that the purpose of the mat is to dissipate high voltages caused static electricity.  The resistor values that you mention will do this quite nicely.