hook_schema install, but not uninstall
I had the same issue and finally I just manually did it in hook_uninstall(), I could find no other recourse.

function mymodule_uninstall() {
  $table_name = 'mytable';
  if (db_table_exists($table_name)) {
    db_drop_table($table_name);
  }
}


Hope this saves someone else some time.