this used to happen to me a bit with my 56k modem as well.
i think it is due to your provider not always capping your
download speed diligently. at times they may give you more bandwidth automatically simply because it is available (for brief periods).
you cannot expect this to be a regular feature, however, and
if you can afford it, get a faster connection (or at least a faster modem. 28k modem?)
best
s