Why did British submarines sink less merchant tonnage?

Why did British submarines sink less German shipping than German U-boats sank Allied shipping in World War I?

British boats had far fewer German targets at sea and often captured merchant ships instead of sinking them.

British submarines faced a much smaller pool of German merchant targets because so much enemy shipping remained in harbor. When a suitable vessel was found, British forces also had reason to capture it and examine or reuse the cargo rather than destroy it. German U-boats operated against a far larger stream of Allied commerce and therefore accumulated more sinking tonnage. Geography, target supply, and operational practice make the raw totals a poor measure of submarine quality.