With Nikon, Exposure Compensation affects the overall exposure, including any automatically metered flash, while Flash Exposure Compensation (when available) affects only the flash's contribution to the overall exposure.

If the scene weren't essentially a flat plane (there isn't much depth to the scene at all), you could use Exposure Compensation to bring down the entire exposure, then boost the flash exposure to correctly expose your main subject. That would isolate the subject from the background. (The opposite can be set in theory, but it won't work unless and until somebody invents a "flash" that subtracts light from the ambient exposure.) More often, though, you'd use the Exposure compensation to compensate for the tonality of the scene (bumping it up for scenes consisting mostly of lighter tones or lowering it for predominantly dark scenes), then independently adjust the flash (usually downward) to add the desired amount of fill.

Note that with these pictures, there isn't much contribution to the exposure from ambient light at all, so if you had set the Exposure compensation to -1 EV and the Flash Exposure Compensation to +1 EV, you would have gotten pretty much the same picture as the normal exposure.