Here, Let's talk about the formula which describe the nature.

Usually we hope to see, as we can see from the science textbook,

v=g*t 

v means velocity, g=32.17 and t is the time

This is
deterministic model
you input something you are sure, and you will get a certain outcome.


Well, but in the real life, we usually find things don't happen in that way, and we have to use a new formula to describe: v=g*t+e

e here means error. That's something you can't expect. So when you pay something, maybe you can't get what you can expect. That's the real life!

This is
stochastic model

So simply, error is the real life!