On iOS (not sure about macOS apps), when you submit an app, it is submitted as "bitcode" (note, this is not "bytecode" - it's bitcode, not a typo). This allows Apple to build your app as needed from your bitcode (think of the bitcode as LLVM IR).

This is done for a couple of reasons: (a) they can take advantage of improvements in their backend compiler (IR->executable) that happens after you submit your app, (b) they can build your app on platforms that didn't exist when you submitted your app

The net result is that the App Store plays an important part in the build process, which could legitimately generate different binaries (even for the same device, see (a) above) for the same app you submit.