USB 1.1 webcam - how do they work with the limited usb speed of 12mb/s?

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Hi,
The Creative Labs Live! VF0400 Cam Notebook Pro is a USB1.1 web cam.
So data transfer is limited to max 12mb/s.
If video is 320w x 240h x 8bits per pixel x 30 frames/sec, this is 18.4mb/s.
How does the webcam push so much data through the limited USB 1.1 bus?
I guess there must be some type of compression, does anyone what compression these webcams do?
Thank you