Carrier Surcharge prices differ for near identical flights?

Was looking at two UC Reward flights, JFK to LHR round trip: same exact outbound flight, but one returns 6 days later, the other 12 days, with both staying a Saturday night.
Different miles requirements, which I get, but the 'Carrier-imposed International Surcharge (YQ)' charges were over $215 more for the second option: $453 on one ticket, $668 on the other. Why?
Made the cost jump from $870 to $1090 in charges for a 'Reward' flight.
Just wondering why the jump in 'Carrier' fees?

Different miles requirements, which I get, but the 'Carrier-imposed International Surcharge (YQ)' charges were over $215 more for the second option: $453 on one ticket, $668 on the other. Why?
Made the cost jump from $870 to $1090 in charges for a 'Reward' flight.
Just wondering why the jump in 'Carrier' fees?