You'll get varying readings depeding on whether or not that particular bit of lava settled over some organic matter and cooked it while the lava solidified.
Radiometric dating is an atomic-level measurement. Melting and re-solidifying it doesn't change it's atomic structure. They're playing an apples-and-oranges game.
Which seems to be a very popular game. The mind grasping for 'any' answer to explain something currently beyond our comprehension.