How about using a different toolchain? (eg: gcc vs clang). Or even different versions of a toolchain? Or a dependency that has to be downloaded? Being able to build consistently requires way more effort than just following professional practices. One method I know is to pin everything that goes into a build - source, dependencies, toolchains, configurations and environment.

Results of non-consistent build can be as simple as a difference in performance. But it could also be a malware injected through the compiler.