USB speeds vs device speed ratings

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The answer to this must be something rather fundamental, but it escapes
me. Perhaps someone is able to explain.

There are several broadband USB modems on the market these days. Mine,
for example, is rated at 2.4mbps. How can a USB device have a speed
that by far exceeds the speed of which USB is capable? Not only that,
but my modem appears to be a USB1 device, which should mean a limit of
12mbps. Something doesn't add up. In reality, I only manage speeds that
occasionally reach a maximum of about 20% of the 2.4mbps rating, but I
suspect that is for reasons other than those posed by my question, but
in any event certainly exceeds USB1 speeds.

To further complicate the question, I can mention that the modem is
shown to be connected to a COM port, and these things have a speed
limit of 115,200 bps. This throws another variable into what I see as
inconsistency in all these numbers.

What am I missing?

Thanks for any input.

.

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