Well, for your flight there is was clearly no need to move pax from Economy to Plus and apparently SK didn't feel it was worth your SEK 700 to put you in Plus and give you the Plus experience; SK would like you to purchase a Plus ticket next time rather than bidding for upgrades.
My bids for intra-Scandinvian flights have also been accepted. I don't use if that often, so my data point hardly say anything about the chances of bids being accepted in general.