Why are the measured values of the resistors different from the nominal values?

A certain tolerance is made for resistors that are sold, though these are not guaranteed to be near to a specific value than that percentage of precision.

Nowadays, resistors are made by loads, sorted automatically and are aimed at a specific value. They run resistors it through an automated Ohmmeter if they try to make one that is 27000 Ohms. They put on a marking that says it is 27000 Ohms + - 20% if it is 20% higher or lower. They put on a marking that says it is 27000 Ohms + - 1%, if it happened to be within 1% of 27000 Ohms. The less accurate ones are sold less than the more accurate ones.