$System.discover_object_memory_usage - why why do all tables appear twice in the list?

I'm running a query against $System.discover_object_memory_usage to view the memory used by each table.

Every table has two entries listed in this view, each with identical OBJECT_PARENT_PATH and OBJECT_ID. What is the significance of these two entries - they have very slightly different values for OBJECT_MEMORY_NONSHRINKABLE, but they are very close.