By default, many compilers include things like local filesystem paths, build server hostnames, or build timestamps into their binary artifacts. These will obviously differ build-to-build.
Even without that, it's possible to accidentally leak entropy into the build output. For example, readdir() doesn't guarantee any kind of ordering, so without sorting the list of files it is possible for a binary artifact (or even tar) to produce different output from the same input.
Even without that, it's possible to accidentally leak entropy into the build output. For example, readdir() doesn't guarantee any kind of ordering, so without sorting the list of files it is possible for a binary artifact (or even tar) to produce different output from the same input.