A stepper motor rotates in discrete steps. You command it to rotate one step at a time. A NEMA-17 motor has 200 such steps to a revolution. If you rotate the shaft yourself you can feel the locations of each of the 200 steps.
The locations of the steps are fixed inside the motor, so after 200 steps the shaft has rotated 360 degrees. The angle between the location within the motor of any particular step N and N+1 may have up to 5% error, but since the sum of the angles between each step add up to 360 you'll be back to where you started from after 200 steps.
The datasheet includes the 5% error figure so that you know how evenly the steps are positioned around the circle.
The biggest problem with stepper motors is that they can miss a step. That is, the motor is at position N and after you command it to step it fails to move to position N+1. This is usually due to the load on the motor being too large -- the motor doesn't have enough torque to move to the next step with the load. This may also be caused by trying to step the motor too fast.
In your table scenario the table can become uneven if one or more of the motors misses a step. Usually, however, if a motor is able to move one step it is able to move through all 200 steps in the cycle if the load doesn't change.
The most common way to detect if a stepper motor has missed a step is to use an optical encoder which can be used to determine the position of the shaft.