Because "first" and "last" don't convey priority. You have to know how the configuration parser works with defaults and overrides. Does the parser stop reading after the first match? Do later configuration directives override previous ones or are they appended to them, and if they are appended, in which order are they evaluated?
First and last are only ambiguous if we make the order ambiguous though. There's no reason for the order to be ambiguous, and if it is then obviously we can't use first/last. I don't think that first/last are either ambiguous or implementation dependent concepts. The other questions are simply other questions, and aren't directly related to whether the ordering is ambiguous or not.
There are plenty of applications that read a set list of configuration files and use either the first or last setting found. I agree with the responder that the last setting usually makes more sense.
