NSFetchedResultsController ignores fetchLimit?

I have a NSFetchedResultsController to update a UITableView with content from Core Data. It's pretty standard stuff I'm sure you've all seen many times however I am running into slight problem. First here's my code:

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];

 NSEntityDescription *entity = [NSEntityDescription entityForName:@"Article" inManagedObjectContext:self.managedObjectContext];

 [fetchRequest setEntity:entity];

 [fetchRequest setFetchLimit:20];

 NSPredicate *predicate = [NSPredicate predicateWithFormat:@"(folder.hidden == NO)"];
 [fetchRequest setPredicate:predicate];

 NSSortDescriptor *sort1 = [NSSortDescriptor sortDescriptorWithKey:@"sortDate" ascending:NO];
 [fetchRequest setSortDescriptors:[NSArray arrayWithObjects:sort1, nil]];

 NSFetchedResultsController *controller = [[NSFetchedResultsController alloc]
         initWithFetchRequest:fetchRequest
         managedObjectContext:self.managedObjectContext
         sectionNameKeyPath:nil
         cacheName:nil];
 [fetchRequest release];

 controller.delegate = self;

 self.fetchedResultsController = controller;

 [controller release];

 NSError *error = nil;
 [self.fetchedResultsController performFetch:&error];
 if (error) {
  // TODO send error notification
  NSLog(@"%@", [error localizedDescription]);
 }

The problem is that initially the store has no entities as it downloads and syncs from a webservice. What happens is that the NSFetchedResultsController fills the table with over 150 rows of entities from the store, which is how many the webservice returns. But I am setting a fetch limit of 20 which it appears to be ignoring. However, if I close out the app and start again with data already in the store, it works fine. Im my delegate i do this:

#pragma mark -
#pragma mark NSFetchedResultsControllerDelegate methods

- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller {
 [self.tableView beginUpdates