Checkstyle reports different issues on different runs

I'm seeing something strange with the checkstyle plugin.

We are using Checkstyle plugin 2.5 and configure it using a checkstyle
file in a jar that is an extension of our pom.

Thus:

<project>
<properties>
<checkstyle-maven-plugin.version>2.5</checkstyle-maven-plugin.version>
</properties>
<build>

<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-checkstyle-plugin</artifactId>
<version>${checkstyle-maven-plugin.version}</version>
<configuration>
<includeTestSourceDirectory>true</includeTestSourceDirectory>
<configLocation>iris_checks.xml</configLocation>
</configuration>
</plugin>
</plugins>
<extensions>
<extension>
<groupId>org.example</groupId>
<artifactId>build-resources</artifactId>
<version>1.0.4</version>
</extension>
</extensions>
</build>
</project>

When I run the following command line:
mvn clean checkstyle:checkstyle && cp target/checkstyle-result.xml .
&& mvn clean checkstyle:checkstyle && diff checkstyle-result.xml
target/checkstyle-result.xml

I sometimes see differences like:
> <error line="12" column="5" severity="warning" message="Missing a Javadoc
> comment."
> source="com.puppycrawl.tools.checkstyle.checks.javadoc.JavadocVariableCheck"/>
279a281,286
> <error line="122" column="9" severity="warning" message="Missing a Javadoc
> comment."
> source="com.puppycrawl.tools.checkstyle.checks.javadoc.JavadocVariableCheck"/>
> <error line="123" column="9" severity="warning" message="Missing a Javadoc
> comment."
> source="com.puppycrawl.tools.checkstyle.checks.javadoc.JavadocVariableCheck"/>
> <error line="124" column="9" severity="warning" message="Missing a Javadoc
> comment."
> source="com.puppycrawl.tools.checkstyle.checks.javadoc.JavadocVariableCheck"/>
> <error line="125" column="9" severity="warning" message="Missing a Javadoc
> comment."
> source="com.puppycrawl.tools.checkstyle.checks.javadoc.JavadocVariableCheck"/>
> <error line="126" column="9" severity="warning" message="Missing a Javadoc
> comment."
> source="com.puppycrawl.tools.checkstyle.checks.javadoc.JavadocVariableCheck"/>
> <error line="127" column="9" severity="warning" message="Missing a Javadoc
> comment."
> source="com.puppycrawl.tools.checkstyle.checks.javadoc.JavadocVariableCheck"/>

I don't change code between runs, so I assume this may never happen.
What could cause this?

With regards,