61mOhm is the calculated current sense resistor value, but a 60mOhm resistor was chosen for the design to ensure the circuit outputs a minimum of 600mA before going into current limit.
If exactly 61mOhms was used the design may go into current limit just as or before the 600mA maximum specified output. It is also important to note that it is important to use a 1% resistor here also.