Uncompressed text files (and other files that can be compressed significantly) can result in reported speeds like that since the modem and PC are compressing the data before it is sent. 33.6 KB/s is probably the throughput (amount of total data being sent) where the actual transmit speed across the phone line is closer to 4 or 5KB/s.
The other possibility is that your system is actually reporting Kb/s and not KB/s. (little 'b' is bits, big 'B' is bytes). 33.6 Kb/s is approximately the same as 4.2 KB/s since there are 8 bits in 1 byte.