Both stocks are ADR's or whatever they call them now. The B share ADR's escape the tax on dividends because they are formed offshore and are exempt from both the Dutch and British tax. The Dutch do not tax B share dividend because the dividend is considered paid in England. The English cannot tax the B ADR's dividend because the money never comes to England. That makes the B ADR's more valuable. From there it is a matter of supply and demand and how much the exemption from a tax on the dividend is worth to the market.
I don't think anyone at any given time can explain the difference between two stock prices and especially RDS since so many variables are in play, like Euro exchange rates, different tax reasons, and so on. I'm still miffed at RD for putting U. S. Shell under their umbrella years ago and it hasn't seemed the same since.