Maybe you are just lucky. I have folding mirrors and the glass rarely returns to the exact position, most of the times going back pointing slightly higher than it was. I guess it depends on the manufacturing process of the memory motor that makes some units more precise than others. As far as I know, there's an analogue resistive memory inside the motor (like a potentiometer). I've measured the resistance of one of those devices and it ranged from 1.2 k to 2.7 k at each end of the vertical movement. It seems that the door module stores the current value before dipping, using a single byte A/D converter. When it has to move the glass back to position it moves the motor while reading the position (again through the one byte A/D converter) until it reaches the stored value. Not a high precision process. I don't know there is a so terrible memory mirror positioning in other models or brands, but I suppose there would be a lot of complaining from owners, especially in models that have multiple memories for seat and mirror positions.