OA min/max input offset voltage
From the datasheet schematic:



You can see a number of things that could cause the offset voltage to change with temperature or with gradients across the die. Vbe of the transistors Q3/Q5 is the big one, but changes in the resistor values with temperature or time or even physical stress on the die could also have effects. 

The typical drift (TCvos) of this amplifier is 2uV/°C over the -40°C to 85°C. Generally speaking, an amplifier with a high initial Vos will drift more than one that happens to be matched more closely out of the tube. 

These days, if you need low Vos/TCVos you should specify an amplifier with guaranteed specifications that meet your requirements in that department.