It's all about psychology as stated above. Literally, because the way your eyes/brain work together to "interpret" images is about psychology.
Generally speaking, set your graphic settings so you get as high an FPS as possible, without making things look too low quality, yet while maintaining over 30FPS. Turn on VSync as well, as this can help a ton with the perception of "smoothness".
Also look at the refresh settings of your monitor, and set it to the maximum it's capable of based on your resolution settings. This will also help.
When comparing your PC monitor to a console game (especially if it's on a TV) keep in mind that the refresh settings between a PC & TV are not always the same. So there's an inherent difference in perception there. As an example, True HD is set in 1080p @ like 24FPS. Yet on an HDTV it looks smooth and beautiful. There is usually a marked difference in resolution as well. Depending on which cable type you use to hook up to your TV (RCA - Analog, HDMI - Digital, or Component Cables) your resolution capabilities vary there (let alone what your TV displays, ie 480i (old TVs), 720p, 1080i, 1080p).
So comparing Console gaming on a TV is to a computer is like comparing Apples and Oranges.
And as Jack insinuated, the distance from a PC monitor is closer than the distance to a TV when using a console to play games. This difference, will create a perception change as well. The closer you are to the image, the more imperfections and "refresh" you're going to notice. Where as the further away you are, the smoother and more blended images will appear. For a good example of this, stop by an Electronics store and check out their HD tv displays. From 10-20ft they look FANTASTIC, but within 1 or 2 feet you can see the pixels on the display.