If the flight it marketed by Delta (has a DL flight number) then you go by DL's fare codes - so G would be Premium Select. Guessing you're on DL operated SEA-LHR in G (premium select)

KL doesn't have a premium select / premium economy so if you rebooked then you'd be in normal econ+ which would explain why it is a 'good deal' since you're going from a real premium econ seat to just a normal econ seat with a little extra legroom.
It should be pretty clear if you actually went and try to book that other routing - you'll probably see it show Econ+ for the KLM operated flight to AMS.