ear dependency jar is wrapped by another jar

Maven project has ear and ejb modules. Ear module depends on ejb module and includes it in result ear file. When deploying ear some kind of ejb dependencies jars are wrapped into another jar. Example of jars: jtds-1.2.2.jar, jibx-run-1.2.1.jar, mvel2-2.0.10.jar. Don't know really why these jars are wrapped into jar package (for example mvel2-2.0.10.jar contains another mvel2-2.0.10.jar package which contains correct jar content) there are many jars that are deployed correctly.