Scope for med moore vs anchoring

In previous lessons you talked about the catenary shape of the anchor chain. You suggested 50 ft put 2x depth for an all chain rode. However, you have gone to 5x depth for this mooring. In a shallow area this would not give enough scope for the anchor to be sitting correctly based on your previous lesson (and you would use more than necessary in more depth). Can you explain why the rules for the scope of the anchor are different when doing a med mooring or with this long line method vs standard anchoring?