How comes the shipping costs is higher in the shop than in the marketplace?
Marketplace and Spreadshop are concepted by two different business departments and also differ in the pricing & commission structure. Both units are testing independently customer behaviours or adapt to provider cost changes. This does not happen in regular but from time to time.
What you see as a difference might be a temporary change and ist most likely only know to you, because you are actively comparing 2 different businesses.