apt-get --print-uris: why filenames sometimes different?

[Dan Jacobson <jidanni@dman.ddts.net>]

$ man apt-get

       --print-uris Instead of fetching the files to install their
              URIs are printed.  Each URI will have the path, the
              destination file name, ... Note that the file name to
              write to will not always match the file name on the
              remote site!...

Indeed, why are some of those names different?  The difference I found
was that sometimes an [0-9]%3a is inserted:

$ apt-get -qq --print-uris dist-upgrade|sed "s=.*/==;s/'//;s/.%3a//"|\
awk '$1!=$2{print $1,$2}' #eliminated the differences with ^^^^^^^^

But why the need to be different?  I'm following
/usr/share/doc/apt/offline.html/ch3.html .
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