The superposition of multiple possible realities. There are an infinite number of universes similar to this one, and in each one the shaving gel contains varying amounts of air. When the probability wave collapses into a single Eigenstate through measurement the different possibilities are smeared together on a macroscale.
For those who ask why this doesn't happen with other things, it's because each universe is unique and in this particular one probability smearing only happens on a macroscale in shaving gel.