The difference in these measurements, if any, is the temperature hysteresis. Any difference in these two room-temperature measurements also represents the temperature hysteresis. The deviation can be positive or negative. After many temperature cycles, you might think that the reference output voltage is far off. Because temperature hysteresis is both positive and negative, however, the deviations resulting from a series of temperature cycles tend to cancel each other out, producing a final average output voltage close to the nominal value.
Typical units are parts per million per volt and percentage per volt.
Line regulation is a dc parameter, and manufacturers typically specify line regulation at dc. Line regulation measures the change in output voltage for two or more dc input voltages and has little meaning if the input voltage varies rapidly, as for a voltage transient. In general, line regulation deteriorates inversely with the rate at which the line voltage changes. For applications likely to have line transients, reference-input capacitors can minimize the resulting variations. Load regulation is a measure of the change in output voltage due to a change in the reference load current.