USP specifies that the measured melting range must
a) Lie within the specified minimum and maximum temperatures (e.g. 81–83 °C for vanillin), and
b) Not be broader than the specified admissible values (e.g. 1.5 °C for vanillin).
So, the permitted measured melting range for vanillin could be 81.0–82.5, 81.3–82.8 °C etc.