You're talking cross-airline redemptions. And yes, the surcharges there follow no logic at all. And point of origin/sale can change things dramatically.
Here, it's redemption of Jet points on Jet metal, where the surcharges (fuel or otherwise) are significantly higher for a return than for two one-ways.
Using your example, it's akin to LH saying:
FRA-JFK = 284€
JFK-FRA = 388€
But FRA-JFK-FRA = 1000€
That situation is a Jet speciality.