The listed FPS of a camera is its theoretical maximum that the camera's image sensor can do, not the actual FPS your computer may obtain.
While the electronics of the image sensor are usually capable of obtaining the full frame rate of the specifications, such things as the processing electronics of the camera, the USB buss bandwidth, overall CPU load, other programs running on your computer, the specific webcam software you are using and the available memory to your camera and software group, will all effect the actual FPS which you will see.
The most important factor effecting FPS is the size of your selected image. The larger the image, the slower the frame rate.
The second major factor in frame rates is how fast your computer can keep up with the data stream coming from your camera. A slow computer, one that has a lot of background programs running on it, or one that has a bunch of devices connected through the USB buss will substantially effect your frame rate.
Assuming your computer's operating system is in pristine condition, (meaning brand new and with no other software installed) and your webcam is the only device using your USB buss, you may be able to obtain the highest frame rate when using the smallest image size the camera allows. However, most webcam software programs just aren't designed to capture images faster than 15 frames per second.
If you have a special purpose which needs a high frame rate with a large image, consider using some device other than a webcam as the source, such as a FireWire (IEEE 1394) video camera, or a special system such as the D-Link DVC-1000, a video phone for the hearing impaired. If your purpose doesn't require a large image then you should chose a USB 2 camera and use a small image.
Basically, don't expect a webcam to perform at 30 or more frames per second, current technology just can't manage it on most computers.
As a matter of interest, unless there is a great deal of fast movement in an image, it is difficult for the human eye to see much difference between 15 and 30 frames per second. Those who were around 20 or more years ago and either shot or watched 8 millimeter home movies may remember that most of those were shot at 15 frames per second. The main reason we use 30 frames as a reference is that is what American TV uses, and that number comes from the fact that we use 60 cycle AC current in the US.