They could do, but I'm not actually seeing any SVG-AMS fares (except IATA level) so was assuming it was constructed as SVG-LHR + LHR-AMS. If it's not sector fares combined end-on-end and SVG-LHR is more than SVG-AMS then yes, the fare would be recalculated first. If that wiped out the residual taxes then that would be the end of it; nothing refunded, nothing charged.