It is correct, the first resin balls were urethane enhanced with a reactive resin additive. To understand the result you have to understand a bit about
Coefficient of Friction. The COF between a ball and Lane surface decreases as the amount of oil increases. Prior to the addition of resin, sanding increased the COF on both the oiled and drier portion of the lane. What resin additives did was increase the COF on the dry without increasing it on the oil. Thus a polished pearl reactive would react stronger in the dry than a non reactive in the dry wuth 5he same surface prep.
Today there are so many different shell materials that it is hard to tell what is going on except for non resin urethane that still responds to surface adjustments in both the heads and coming off the oil