Obama did not pardon Chelsea Manning. He commuted a sentence of 35 years to 7 years.

Trump did not consult with the Justice Department before pardoning Mr. Arpaio. Indeed, the former sheriff had not even applied for a pardon through the long-standing Justice Department system. The department’s guidelines state that pardon applicants should show remorse and atonement for their actions. Mr. Arpaio has shown none. Arpaio broke his oath to uphold the law, and now Trump has put him above the law.

Trump first tried to have the department drop the case against Arpaio. AG sessions had to tell him that's inappropriate. But it gives you an interesting view into Trump's mind and how seriously he takes his oath to uphold the law. People he likes get out of jail free, people he doesn't like, "LOCK HER UP!"