No matter what offset you put there, it will always returns same result.
https://flightxml.flightaware.com/json/FlightXML3/FlightInfoStatus?ident=N8545V&filter&howMany=10&offset=10
https://flightxml.flightaware.com/json/FlightXML3/FlightInfoStatus?ident=N8545V&filter&howMany=10&offset=20
https://flightxml.flightaware.com/json/FlightXML3/FlightInfoStatus?ident=N8545V&filter&howMany=10&offset=30