xerces versus classpath versus webapps

[bimargulies]

This isn't CXF-specific, but I bet one of you has been here before.

I have a webapp. It has a JAX-RS CXF service. That service takes an XML file
and parses it using standard JAXP APIs. It sets up XML Schema validation.

xerces is listed as a dependency, no scope specified.

Run as a unit test.

Deployed with mvn tomcat:run, the validation rejects the input with a bogus
complaint. Backtrace shows that we're using the Sun repackaged JAXP, not
xerces.

I feel as if I must be missing something simple, but what?