What does abnormal anatomy mean?

It typically refers to anatomical structures that for whatever reason, can be found in different places than would be considered normal.
For example, for the majority of people, the heart can be found just slightly to the left of the sternum. However, in a condition known as dextrocardia, a persons heart may be found to the right of the sternum instead; as a mirror image of a normal cardiac anatomy.
These types of abnormalities are rare, but sometimes may interfere with other systems, or have the potential to result in missed diagnoses etc.