It followed a request for information from the force’s Counter Corruption Unit (CCU) after an information breach from a live murder investigation, Mr Richardson said.
Last month the Interception of Communications Commissioner’s Office (IOCCO) found Police Scotland had broken data rules five times in its attempts to unmask a journalist’s source.
The Iocco said the force had been “reckless” in failing to obtain judicial approval when attempting to access communications data.
In a heated session of the Scottish Parliament’s justice committee, Mr Richardson said media coverage had painted him as an “archetypal villain” who is “playing fast and loose” with the rules.
He said the reality was “very different”.
Officers from the CCU began investigating after information appeared in the media about the investigation into the murder of Emma Caldwell in 2005.
Mr Richardson said Detective Superintendent David Donaldson - who is not part of the CCU - was asked to look at authorisations and tried his “level best” to apply the guidelines.
However, he misinterpreted recently introduced guidelines protecting journalists from the Regulation of Investigatory Powers Act (Ripa).
Mr Richardson said: “The suggestion that we have not been careful is not accurate.
“It just so happened that the officer concerned has misinterpreted brand-new legislation, that at the point he was asked to do this was 22 days old.
“I don’t seek to minimise this. A mistake was made, but the context of the mistake is important. This was an officer was trying his best to do his professional duty and has made an error of judgement.”
He added: “This is not an officer who has simply thrown caution to the wind - nothing could be further from the truth.”
Mr Richardson said suggestions the CCU were “running amok” and doing things that were “inappropriate” were not “simply not accurate”.