From a previous post of this subject,, not by me but nevertheless.ps..apologies for the paraphrasing!
e. The reason given why Vref Ice is applied to the NG and not to the Classics is that they have different wings. The Vref Ice policy was because when Boeing was doing initial flight testing, they put artificial ice on the wings. The FMC automatically compensates for the change in performance for the ice at flaps 30 and 40, but was not sufficient to compensate for flap 15, so an extra 10 knots has to be added for performance, and to keep the Ref speed out of the amber band.
The problem with the ice was not the wings but the empennage.