It is how the mirror modifier behaves, it depends if you move in object mode or edit mode since it takes it queue from the object origin. You can learn all about modifiers in this course:
https://cgcookie.com/courses/course-modify-a-complete-guide-on-blender-s-modifiers-84c3c6a66db79804
I strongly recommend you pause the Press Start course and start watching the fundamentals instead.
