> What do these messages mean?  What causes them, and is 
> there something I should do differently so they don't 
occur?
During an update, the html file existed on disk locally, 
but the original html data could not be found in the cache. 
This means that the cache was partially flushed or erased. 

It generally happends when you break an update process 
before the end, and before the previous-last-link-position. 

Example:

first download:
1:  index.html image.gif foo.html bar.html (end)
in cache: index.html image.gif foo.gif bar.gif

update:
2:  index.html image.gif foo.html bar.html (end)
in cache: index.html image.gif foo.gif bar.gif

second update, interrupted:
3:  index.html image.gif (break)
in cache: index.html image.gif

third update:
4:  index.html image.gif foo.html bar.html (end)

The foo.html and bar.html will not be found in the previous 
cache, but will be found on disk.

> 13:28:22 Warning:  HTML file (37429 bytes) retransfered