Why does the prusa i3 sometimes put the layer more right or left as they should be?
I'm not sure if I read your question correctly, but if I do, what you are referring to is called "layer shifting" and looks like this:



This happens when the stepper motors fail to perform a step upon receiving the signal from the firmware.  Since stepper motors do not have any way to know their actual position (differently, for example, than servos) they will keep on printing as if nothing happened, and thus the subsequent layers will be shifted of the amount of steps they missed to perform.

Moving on to why this happens in sth...  I don't own a MK3, but I take it that sth is short for stealth mode, the new silent mode that has been widely showcased in reviews and articles on the printer.

That mode of operation is made possible by the Trinamic TMC2130 stepper drivers.  Normally these drivers monitor the power consumption of the steppers and are capable of deduce a missed step by sudden changes in that.  If they do, the MK3 will actually re-home the X and Y axis to fix the problem and resume printing normally.

However when operating in their StealthChop mode, the TMC drivers provide less energy to the motor themselves (to keep them operating quietly) and - more importantly - are unable to detect missed steps.  Less power will make much more likely that any sort of resistance to the print head movement will cause missed steps, the absence of detection will cause the printer to not even notice and cause layer shifting.

Again, I don't own a MK3 and I have no direct experience with it, but I would suggest updating the firmware to the very last version: I read in an article some weeks ago that a recent version addressed exactly your problem, by progressively increasing the power output along the Z axis (as the likelihood of layer shifting increases with the height of the print).