Not sure what version this changed on, but when you select multiple files and Action > Delete Files, it deletes ALL the files in view not just the selected ones.
Bug?
Deleting Individual Files
What has changed (and this changed a while ago) is that selection is based on what in the first rowno column, not the main table, if nothing is selected it is applied to all visible rows. This is how all the tasks now work (not just delete files) and means the main menu and toolbar can be used to act on both all files/selected files without needing two seperate options/shortcuts.
Tasks that need a particular field selected in the main table such as Copy/Paste/Undo ectera work on what is selected on the main table.
Sorry for the confusion.