Maybe because of some errors that happened during the experiment
1 2 1
what is the equation for the calculation??
its not easy to meet the real answer when it involves physics...there are too many possible errors that can happen ..or maybe the calculation is wrong..maybe forget to change any units
may be friction trolley n plane could affect the result
yes..it could be
The real value is when there is no other factor affecting it, for instance, air resistance. That is why for some questions regarding experiments will ask you to assume there is no air resistance acting on the object and so forth. 

To obtain the real value needs complicated experiments. Only the scientists who discovered, studied and comprehend the theory entirely know what to alter and what to do in order to do it.