Predictions based on resistance measurements not matching reality?
Your tongue is more sensitive to electrical current than your fingers. You might find the current is higher than 6uA too with the higher voltage (linearity is not guaranteed). Frankly 1.5M seems extremely high for a wet mucous membrane- more in the range of skin resistance. It will also vary with the spacing (and area) of the electrodes. 

Edit: I tried it with a multimeter and got more like 50K. 

You can put the multimeter in series with the battery and actually measure the current from the 9V battery directly. Add a 10K resistor or something like that to protect the meter.