When you open build.xml under the "file" menu. if you double click the entry,
you open build.xml in the project directory. If however, you click the little
plus icon and open up all the targets, and double click one of them, you get
build-impl.xml from somewhere completely different.

This is completely counter-intuitive, and causes extreme confusion. Whilst I
admit it is a mistake that if made once, once it has been explained, you never
make again, it takes a lot of posting to get it explained.

It would probably be better if build.xml had blank entries for the customisable
targets, and clicking the little + icon gave you a list of the customisable
targets, and if you wanted to edit build_impl.xml, you should go through the
file menu to find it.