The description of the xjc ant task is not up-to-date.
In section 2.2 (http://jaxb.java.net/2.2.6/docs/ch04.html#section-356252324237826) the "target" parameter attribute is described as "defaults to "2.1". This should be 2.2.
The xjc compiler behaves differently when you leave out this parameter (as it is not required) compared to setting the parameter to "2.1". (I also checked the code, the default is "2.2" for the version included in JDK 7.
The same - probably - holds true for the "source" parameter. (Didn't check that one.)