LDR exceeding specified dark resistance
The datasheet says that it is MINIMUM 1.0&nbsp;Mohm after 15 seconds after removing the test light.

The test procedure isn't laid out explicitly, but it seems that they exposed the unit to a light of a specific intensity (1 foot candela) for 16 hours and then measured the resistance for light. They then shut off the light and measured again after 15 seconds. In those conditions you are guaranteed 1&nbsp;Mohm. That doesn't mean that the dark resistance can't be higher, just that it won't be lower.



Additional tips from Spehro Pefhany's comment:
You can limit the maximum dark resistance by putting a resistor in parallel to the LDR.  

If you want it to top out at 1&nbsp;Mohm, put a 1&nbsp;Mohm resistor in parallel to the LDR. The parallel combination then has a combined resistance of just under 1&nbsp;Mohm in complete darkness, and (assuming the 12&nbsp;kohm max light resistance from the datasheet) a light resistance of 11.85&nbsp;kohm.