NIST ruled out neutrinos as "the" factor,so it remains inexplicable, atm.<quoted text>
As pointed out in the article, it is probably due to interaction with neutrinos from the sun. This is supported by the particular isotopes of Cl used. It is the specific isotope that is used to detect electron neutrinos.
But we will eventually figure it out, we always do.
It's thought a yet undiscovered particle.