区别就是前一句他有几个兄弟，要用 who [that] is a doctor 来区别。后一句他只有一个兄弟，所以不需要 restrictive clause 来决定是哪个。
不过这是很不好的例，在实际说话时没有这样的分别的。这样才是真正的 restrictive clause 与 non-restricive clause 的例：
Alan takes care of pumas that have been abandoned by their owners when they grew too big.
Alan saw a puma, which was obviously abandoned by its owner, prowling in the neighborhood.