You're either assuming that "build environment" includes more things than the author does (it's not exactly a well-defined term), or you're not aware of the sources of non-reproducibility.

It can be simple things (something includes timestamps in binaries, which can be worked around by pinning the time), and complex things (concurrent linking of binaries is done FIFO-style, and different compilation units finish at different times in different runs because of varying processor load).