Aah, I think I've realised what might be causing it.
I have the blocks set like this as I cycle through the menu:
Mon-Fri
1730-2000, 0630-0930, 1230-1430
So the blocks are not in time order.
Mon-Sat
1730-2000, 0630-0900, 0900-0900
I wonder if this confuses it. It turned out this way...