"I'd like to know how this was possible."
Here's your answer:
Union General Ulysses S. Grant was a slaveholder of record. He refused to give up his slaves until the passage of the 13th Amendment. The Grant’s wife Julia confirms having slaves through 1863 as she wrote in her Personal Memoirs, that:
"Eliza, Dan, Julia, and John belonged to me up to the time of President Lincoln's Emancipation Proclamation. When I visited the General during the war, I nearly always had Julia with me a nurse. She came near being captured at Holly Springs"
One of Grant’s slave’s name was William Jones. In 1858, while attempting to make a go in civilian life as a farmer near St. Louis, Missouri, Ulysses S. Grant bought the slave, William Jones, from his brother-in-law. Grant's also became the owner of record of his wife’s inheritance of four slaves, but as was the case at the time, women could not actually own slaves, so they were under the control of Grant. No record has been found of these slaves having been freed prior to emancipation in Missouri in 1865.
http://www.scv674.org/SH-3.htm
I would answer that January 1863 is not December 1865. You post a quote from Mrs. Grant's memoirs stating she freed her slaves early in 1863. There is no record of Mrs. Grant being seen with any of her slaves after early 1863. When she visited Grant with her children during the Petersburg campaign she brought a hired German girl as nurse and not one of her slaves. And you also ignored the fact that Missouri emancipated all slaves in January 1865. So I will ask once again in the face of all evidence to the contrary, how can the claim that Grant owned slaves until the adoption of the 13th Amendment be true when neither he nor his wife were living anywhere that slave ownership was legal and there is no evidence at all of their being present any time after January or February of 1863?
In 1858, while attempting to make a go in civilian life as a farmer near St. Louis, Missouri, Ulysses S. Grant bought the slave, William Jones, from his brother-in-law. No record has been found of these slaves having been freed prior to emancipation in Missouri in 1865.
On the contrary, Grant's memoirs state, and Missouri records confirm, that ownership of William Jones was transferred to Grant from his father-in-law and not his brother-in-law. Grant's memoirs further state, and Missouri records also confirm, that Grant emancipated Jones in 1859 prior to moving to Illinois, not 1865 as you claim.