Maven: variable interpolation in POM

In POM we can have ${project.version} or special variables like ${project.basedir}.

This example at the very end of Introduction to the POM accesses project.properties.mavenVersion by just writing ${mavenVersion}.
Why is that sufficient? I thought we need ${project.properties.mavenVersion}.

In general, when are we allowed to access a variable without fully qualifying it using${project.xxx.variableName}?

<project>
  ...
  <properties>
    <mavenVersion>2.1</mavenVersion>        <----- variable defined here
  </properties>
  <dependencies>
    <dependency>
      <groupId>org.apache.maven</groupId>
      <artifactId>maven-artifact</artifactId>
      <version>${mavenVersion}</version>    <----- variable accessed here
    </dependency>
    <dependency>
      <groupId>org.apache.maven</groupId>
      <artifactId>maven-project</artifactId>
      <version>${mavenVersion}</version>    <----- and here
    </dependency>
  </dependencies>
  ...
</project>

project. is the recommended form to use for all project variables except user defined (<properties> section) ones.