Why heat of neutralisation from experiment is differ from theoretical value?

All resistors are manufactured with a built in tolerance level signified by the final colour band on it. These range from 0.01% to 20%. As the 20% one are much cheaper these are the ones that are more often used on not-critical applications and schools. This means that although they will normally be within 1%, a 1k resistor can be anywhere between 0.8k and 1.2k