This probably has something to do with the pom file here: https://github.com/jacoco/jacoco/blob/master/jacoco-maven-plugin.test/pom.xml
It is using a plugin called the maven invoker.
The Invoker Plugin is used to run a set of Maven projects. The plugin can determine whether each project execution is successful, and optionally can verify the output generated from a given project execution.
And if you read about filtering files with this plugin, it mentions:
POM files selected by the include/exclude patterns. The tokens to be filtered must be enclosed in @...@ to avoid interferences with Maven's normal POM interpolation, i.e. use @project.version@ instead of ${project.version}.