Debian seems to only considers a build reproducible if the entire .deb file is byte-for-byte identical, not just the ELF files. A bunch of the packages they're tracking are flagged for documentation files such as PDFs not coming out identical.
It's a good question! It's in fact such a good question that the reproducible builds folks had to draft a solution for it: the SOURCE_DATE_EPOCH specification. Reproducible builds read the timestamp of the last debian/changelog entry (you can imagine the last git commit or something, for non-Debian packages) and pass it through to the build system, so that the "current" time in tarballs etc. is actually the time of the changelog entry.
The "build number" is actually the version, which is determined from debian/changelog. A hash shouldn't change when its input doesn't change. Timestamps can be faked by intercepting system calls.
I'm sure there are many, many more techniques involved in making a build reproducible. Hey, they achieved >90% already!
The best approach that I have found is to use the git commit timestamp (or epoch 1 as a fallback) instead of the current timestamp. After all if the build is reproducible, it doesn't matter when it was built, only what input it was given.
