'Splain?
After Lee declined Lincoln’s offer to lead the Union Army, he left Arlington House and many of its possessions behind as he joined Virginia...
The union troops soon took over Arlington House and ‘captured’ most everything.
Some of the collection were broken, stolen, disappeared...There never has been a final accounting that I know of. Even now, some families may have a nice piece with or without knowing what they have.
His wife ( and even her father : G W Parker Custis ) apparently gave away or sold some pieces.
Mount Vernon apparently received many of their current pieces from the collection of Governor Caleb Lyon ( who was a frequent mid-19th century visitor to Mt Vernon and Arlington House ).