The short answer You are running the command in one display, and fbset is telling you about another one. These two displays are the framebuffer, which runs the tty? CLI screens, and the display manager, which provides your Gnome session.
The longer answer The framebuffer is used while you boot, and for the text consoles you typically get to with Alt-F1, F2, etc.
Gnome is a display manager that also provides an X server for graphics applications. They are mostly independent of one another, but in most systems they do share a common "Direct Rendering Manager" or DRM driver. This allows you to swap between them without having to reset the video hardware or see strange graphics garbage (like we had to do years ago when X-servers ran completely in user space and talked directly to hardware). It also allows for a nice smooth transition from the framebuffer-based boot screen to the display manager-based greeting screen.
You may find that when you Alt-F2 to get to the framebuffer tty2 console, then login and try playing with fbset it will make a lot more sense. Although I need to conclude with a bit of warning. It may still not work like you think it should. Many modern frame buffers don't actually change the hardware resolution, but only change a "window" against that screen. So, you can select a lower resolution in fbset, but it will not make the images larger, it will only limit the text output to a smaller block aligned to the top left of your screen. If someone could answer how to get that to work properly, I'd be very interested.
If you really want to dig deeper, then check out this site. There is a nice picture that ties everything together there.
