The "select all" should work for all results and not only the datasets being displayed in the first search.
The use case considered would be:
1) refine the search until all required datasets are gathered
2) use "Select all" to *default* to "selected"
3) unselect anything if required
4) proceed as usual

This would invert the default "Unselect All" since selecting >40 datasets is difficult. It will revert the paradigm and it will work quite nice if "deselecting" <40 datasets.

The implementation should be simple enough.