HTTP has a simple way to say "you already have this": the server sends an
ETag or a Last-Modified header, the client sends it back next time, and the
server answers 304 Not Modified without a body.
Some servers send neither, so every client downloads the whole feed every time. Others regenerate the zip on a schedule, so its date and ETag change every day while its contents do not.
The law
The URL answers with no ETag and no Last-Modified, or on 3 of the last 7
daily checks it sent new validators with byte-for-byte the same file.
How to get pardoned
Serve the feed as a static file: every web server sets both headers by itself. Only replace the file when its contents change.