On page 250 of his report, special counsel Robert Hur explains why it’s not inconsistent as a legal matter for the federal government to pursue charges against Donald Trump but not Joe Biden in the respective cases arising from their classified document retention after leaving office:
With one exception, there is no record of the Department of Justice prosecuting a former president or vice president for mishandling classified documents from his own administration. The exception is former President Trump. It is not our role to assess the criminal charges pending against Mr. Trump, but several material distinctions between Mr. Trump’s case and Mr. Biden’s are clear. Unlike the evidence involving Mr. Biden, the allegations set forth in the indictment of Mr. Trump, if proven, would present serious aggravating facts.
Most notably, after being given multiple chances to return classified documents and avoid prosecution, Mr. Trump allegedly did the opposite. According to the indictment, he not only refused to return documents for months, but he also obstructed justice by enlisting others to destroy evidence and then to lie about it. In contrast, Mr. Biden turned in classified documents to the National Archives and the Department of Justice, consented to the search of multiple locations including his homes, sat for a voluntary interview, and in other ways cooperated with the investigation.
So far as I can see, there is one standard of justice at work here, not two.