And you know this because you know where you aren't. By subtracting where you are from where you aren't, or where you aren't from where you are (whichever is greater), you obtain a difference, or deviation. You use deviations to generate corrective commands to drive yourself from a position where you are to a position where you aren't, and arriving at a position where you weren't, you now are. Consequently, the position where you are, is now the position where you weren't, and it follows that the position where you were, is now the position where you aren't.

In the event that the position where you are in is not the position where you weren't, you have acquired a variation, the variation being the difference between where you are, and where you weren't. If variation is considered to be a significant factor, it too may be corrected by you. However, you must also know where you were.

Your guidance scenario works as follows. Because a variation has modified some of the information you have obtained, you are not sure just where you are. However, you are sure where you aren't, within reason, and you know where you were. You now subtract where you should be from where you weren't, or vice-versa, and by differentiating this from the algebraic sum of where you shouldn't be, and where you were, you are able to obtain the deviation and its variation, which is called error.