OS name and version determination relies on /System/Library/Frameworks/JavaVM.framework/Frameworks/JavaRuntimeSupport.framework/JavaRuntimeSupport. That is not installed from *.dmg package and can be absent in system.

src/solaris/native/java/lang/java_props_macosx.c:
       38 // need dlopen/dlsym trick to avoid pulling in JavaRuntimeSupport before libjava.dylib is loaded
       39 static void *getJRSFramework() {
       40 static void *jrsFwk = NULL;
       41 if (jrsFwk == NULL) {
       42 jrsFwk = dlopen("/System/Library/Frameworks/JavaVM.framework/Frameworks/JavaRuntimeSupport.framework/JavaRuntimeSupport", RTLD_LAZY | RTLD_LOCAL);
       43 }
       44 return jrsFwk;
       45 }

So, system properties os.name and os.version become 'Unknown' for that system.