Everything looks normal to me. If I was to speculate, I'd say that you've inadvertently (or not) selected dummy booking dates which don't cross a Saturday night on this occasion. Outbound at c£550 and inbound at c£2690.
If you maintain those dates and check a dynamic package price, you'll see the saving you quote, as the Saturday night stay rule doesn't apply in such circumstances.
Is that a possible explanation?