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?

[mitchja]

It’s the new dynamic pricing system VS are now using.

YQ is not a fixed fee and because YQ is just a made-up fee the airlines came up with themselves, it can be set to anything they want.

Thanks James, but wow!

I got that the miles would go up and down with the new dynamic model, but the random 'Carrier Imposed Surcharge' does too? Oof.

Wonder what they'll come up with next?!

One of the primary reasons VS introduced the new dynamic pricing is that Shai, in his infinite wisdon, thinks he can gain another $50m per year from US based customers purely on points redemption value & extra YQ fees.