In short : object positions are (usually) represented using
floating point data types ( google for IEEE-754 on wikipedia ).
The thing with floating point data type is that when the
numbers are big, they lose precision.
In computer graphics this starts to occur when object position coded in single precision floating point is more than 30 000 units ( usually meters ) from (0,0,0) point.
And it starts to look like the position of the object is "quantified".
Look at the before mentioned video at 4:05 - the rifle looks like
it moves for a certain number of centimeters every frame - its
movement is quantified.