Default version of maven-compiler-plugin

I'm using maven 3.2.1 and when I run maven with -X I notice that version 2.5.1 of the compiler plugin is used:

However, if I run

    mvn help:describe -Dplugin=compiler

I get:

Name: Maven Compiler Plugin
Description: The Compiler Plugin is used to compile the sources of yourproject.
Group Id: org.apache.maven.plugins
Artifact Id: maven-compiler-plugin
Version: 3.1
Goal Prefix: compiler

What is the rationale for Maven using 2.5.1?

John