The following is from 56K Modem Info (while written specifically about 56k modems, much of this quote applies to the Classic's 33.6k modem as well):

When you surf the net, the speed at which you receive(download) files depends on several things, the computer that the files are located on, the data links between that computer and your ISP, your ISP's own computers and modems, and finally your computer and modem. When you dial in and your modem says it's connected at say 46K, this means that if the system is working at peak condition you can transfer 46,000 BITS(not bytes) per second. Unfortunately, most of the time there will be a slow link in the system and this will limit the speed which you can download. For instance, if you are downloading a file from an FTP site running on a private PC connected to the net by a 33.6K modem you will only get a maximum of 33.6K download speed. If you ISP's system is slow this will also affect your downloads.

Data traveling between modems is sometimes compressed to fit "more into less" bandwidth. This is explained theoretically here, in a quote from Speeding Up Your Modem Connection (the "polygon" referred to is just a particular brand of modem):

First, the two modems negotiate a Line Speed. The modems will attempt to establish a Line Speed of 28,800. If the quality of the telephone lines between the two modems can support this, a Line Speed of 28,800 is established. If the quality of the telephone lines can not support a reliable connection at 28,800, the modems drop down one speed level to 26,400 and begin the negotiation process again. This process will continue until a reliable Line Speed can be agreed upon (28,800, 26,400, 24,000, 21,600, etc.). After a Line Speed is established, the modems then negotiate which Data Compression and Error Correction scheme to use.

Once Data Compression and Error Correction is established, you can then achieve these higher speeds. What will happen is that information will leave your computer and travel to your modem at a speed of 57,600. Your modem will compress this information down to 28,800 and transmit this compressed information to the polygon modem at 28,800. The polygon modem receives the compressed information at 28,800, then decompresses this information and sends it to our main computer at 57,600. Thus, even though you only have a 28,800 modem, in this example, you have achieved a speed up to 57,600.

The increase data transfer rate achieved by compression of sent data does not mean that your modem is "talking faster," or run at a higher bit rate, but rather that the data sent requires fewer bits for the same sent data, due to compression, so more fits into less.