They probably combined the creepier traits of Mirra and others into Lefty's character.
Most of the scenes in Goodfellas where Tommy and Henry are hanging out are really based on Henry and Paul Vario's sons. Henry Hill was afraid of Tommy DeSimone and rarely hung out with him socially. They only went on jobs together. Either Pileggi or Scorsese decided to combine the Vario kids and DeSimone into one character because it would play better for the screen. These movies are only "based on" actual events, not meant to be accurate documentaries.
"...the successful annihilation of organized crime's subculture in America would rock the 'legitimate' world's foundation, which would ultimately force fundamental social changes and redistributions of wealth and power in this country. Meyer Lansky's dream was to bond the two worlds together so that one could not survive without the other." - Dan E. Moldea
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