When the OS X install happens, the plist is created based on the template. Since it's a new file, it will inherit the umask of the creating shell. When you're running as root, it's not a huge issue, as it's generally already 022, but if you do it some other way (like executing it from a java process) then the install will fail if you're attempting to install a daemon, as Apple specifies it must be root:wheel and 644 for security reasons.
The default posix service sets executable permissions, so it makes sense for the OS X version to set permissions. I've attached the relevant file for this change.
File is from 11.11-stable