Indeed interesting info! I wouldn´t have thought that the loss in pressure would be so dramatic! One possible explanation would be that the advertised spring pressure exceeds the the tensile modulus (
http://en.wikipedia.org/wiki/E-modulus) of the material used.To put it simple the spring pressure is calculated in such that the steel returns to its original strength after "stretching" without loss of tension.This value is the same no matter what steel is used,whether its a cheap steel or a high tensile alloy just as the expansion rate is the same if they are of the same diameter.The diameter of the steel is the key and base for calculation - not the alloy!! If the force applied exceeds that value (calculated in mm˛),the material will wear quickly.Hence,the advertised spring value is higher than what actually should have been calculated.