Rcov coverage question

I am using the following Rake task in a rails project:

http://pastie.org/332921

Can someone explain to me why running “rake test:coverage” seems to run
tests for all gems and libraries used by my app instead of just the
tests that are in my test/** directory?

For instance, RedCloth, gravtastic, builder, stomp and xml-simple are
out output in the coverage html data instead of just the app tests?