Actually all offsets are relative to some point, just this point not always at the start of the file. If concatenate the ZIP file to other file, the unzip utility and the zipfile module are able to infer the starting point and correct offsets. Thus there is no matter what is the starting point of offsets. But Go's standard archive/zip package works only if offsets are relative to the start of the file. I don't know how common such interpretation however.