/bin/ksh: bad interpreter: No such file or directory

I have script with #!/bin/ksh in the first line. When I try to execute this script (run ./myscript.sh) the error occurred: -bash: ./myscript.sh: /bin/ksh: bad interpreter: No such file or directory But when I execute this script through source myscript.sh or bash myscript.sh command - script runs successfully. Yes, ksh is not installed and it is correct to install this. But I can't understand different behavior ./ and bash or source