The FG outputs twice the setting because it is expecting to be connected to something with 50 ohm input impedance forming the 2:1 voltage divider. Has nothing to do with reflections which are not going to occur at DC. There is more to impedance matching and reflections with AC signals but you don't need to go there to understand the voltage divider principles involved at DC and low frequencies.
Actually... the counterintuitive part arises because you are implicitly used to reflections, and don't know it. It's not that the effect is counterintuitive, it's that your intuition is conditioned upon a different experience.
If you run the analysis, you see that DC into an open circuit is 100% reflected, at all times, regardless of the length or impedance of transmission line attached -- the difference is, reflection and mismatch (typically a gross mismatch, going out of ones' way to make good voltage sources and all!) is standard design practice.
From a conventional perspective, the reason the FG's output is "double" voltage is because they
start with that and add the resistor after.
However, from a transmission perspective, an alkaline cell, for example, is, say, 0.2 ohms and 11 watts. It is constantly delivering that power at all times, however, by leaving it connected to a high impedance termination (an open circuit), all the power is immediately reflected back, the superposition of which (reflected and incident waves) causes the appearance of 1.5V as measured on the terminals.
(Shorting it causes an inverse reflection, resulting in zero terminal volts, 7.5A, and all 11W being dissipated in the cell.... it heats up. A matched load of 0.2 ohms draws 5.5W, i.e., 0.75V and 3.75A, and the cell still gets warm, though not as bad as the shorted case.)
Another example: a switching supply normally connects an inductor to a voltage source, then to a load capacitor, alternately (a crude but sufficient description of one type of converter). In transmission terms, the inductor has impedance, and carries reactive power; when the load is light, the reactive power is reflected -- recycled -- through the source. When the load is heavy, the source delivers power into the reactance, and then into the load. (Taking a specific example, this is most apparent in bridge type converters, where the series inductor carries a triangular current waveform; this current doesn't deliver any output power, it's just reactive power cycled into the power supply rails. Under light load, the inverter is loaded only by this ripple; lots of power is reflected, so overall power consumption is small. Under heavy load, little is reflected.)
Tim