Excellent posts are Bill’s here.
Just for the others here, it does seem that even Jack Ruby was at a prior press conference and corrected Sherrif Wade about Lee Harvey Oswald being in the “Fair Play for Cuba” group on Thursday night, November 22nd the day of the assassination! That raises questions about how Ruby knew that and what happened on that following Sunday when Ruby did gun down LHO.
Wikipedia reads:
“In addition, WFAA-TV (Dallas) and NBC newsreel footage show Ruby impersonating a newspaper reporter during a press conference, at Dallas Police Headquarters, on the night of the assassination. At the press conference, District Attorney Henry Wade said that Lee Oswald was a member of the anti-Castro Free Cuba Committee. Ruby corrected Wade by stating that it was the pro-Castro Fair Play for Cuba Committee.” - http://en.wikipedia.org/wiki/Jack_Ruby#JFK
Ruby was a police groupie. He was frequently at police headquarters and well known. He loved being around cops and wanted to be liked by them. He wasn't pretending to be a reporter. Many of the officers there knew who he was.
He knew the correct terminology because he'd been obsessed by the killing since it happened. He read or saw the news reports. He thought of nothing else. It threw his whole state of mind.