Most likely integration issues. When booking SAS TATL fares, SAS will sell DL domestic segs as either SK codeshares with an SK booking class or DL coded flights with DL booking class. If the DL flight is DL coded, there's a $100 fare surcharge. When the flight is DL coded, it will book into a regular DL Main (not BE) class. The SAS fare basis code identifies whether you have a non-refundable Go Smart fare (ends in 'ES'), or a refundable Go Pro fare (ends in 'EF'). You can't identify fare type just from SK booking class.