Why are the call signs of some airlines completely different than their name?
I found this on Wikipedia: 


  Some call signs are less obviously associated with a particular airline than others. This might be for historic reasons (South African Airways uses the callsign "Springbok", hearkening back to the airline's old livery which featured a springbok), or possibly to avoid confusion with a call sign used by an established airline. Companies' assigned names may change as a result of mergers, acquisitions, or change in company name or status; British Airways uses BOAC's old callsign ("Speedbird"), as British Airways was formed by a merger of BOAC and British European Airways. Country names can also change over time and new call signs may be agreed in substitution for traditional ones.


Before America West bought US Airways, their callsigns were Cactus (associated with the deserts of the American West) and USAir. After the merger, the combined airline kept the better-known (airline) name, and the other airline's callsign. Now, after the acquisition/merger of American Airlines, they're using that airline's name and callsign.  Leading to more than a few radio calls recently (late spring / early summer 2015) along the lines of


  "Center, Cactuuu...merican 123, FL 350"