The OP needs to get the simple
voltage divider aspect of Impedance matching first. I had a hard time getting this very simple concept in the beginning also.
When set for 1V the FG actually outputs 2V into its internal 50 ohm output resistor (50 ohm output impedance). It does this because it expects the instrument it is driving to have 50 ohm input impedance (a 50 ohm resistor to ground internally) this forms a 2:1 voltage divider and the driven instrument sees 1V.
The added 50 ohm termination is in parallel with the scope 1meg input resistor and that gives 49.9975 ohms input impedance that is very close to what the FG is expecting.