Your question is well answered by /u/sowser in this thread. To give you the brief summation is that while there were a few free blacks who owned slavers in the proper sense, in most cases, they owned slaves in only the most technical of sense, and what this statistics mostly speak to is black families where one (likely the pater familias) is free, and the rest is legally enslaved due to legal reason sower enumerates. So while a free black man might own 5 slaves, they are likely his wife and four children.