Melting Point of Pure Gold
As with any experimental measurement of physical properties, the techniques involved improve with time, and sometimes new, better ones appear. That also means some values for some physical properties sometimes are slightly adjusted to reflect these better measurements and less error.

In some very rare occasions, the basic definition of some measurement is changed, like the recent change in the definition of the kilogram (see https://en.wikipedia.org/wiki/Kilogram), and it sometimes reflect in some minute adjustments made to another measurements based on the old definitions.

In other, more mundane measurements, the standard (for the measurement) is based on a set of conditions (pressure, atmosphere composition, purity, and a lot of other parameters) and a specific procedure for the measurement that sometimes change in newer versions of the procedure, sometimes resulting in a slight change of the expected measured values.

That doesn't mean the material changed, just that the observed value for the physical property using the described set of parameters and procedures were adjusted.