When relying on Julia Grant's memoirs one has to remember that they were no published until about 70 years after her death. They are open to some question.
Still it is ironic that Lee freed his slaves without compulsion and Julia Grant apparently did not.
The slaves were freed because George Washington Parke Custis's will required them to be freed. That seems pretty compulsive to me. If Missouri records are correct, the Dent family freed their slaves early in 1863, two years before the Missouri Constitution required the do so.
Exactly. She may have freed them afterwards.