The mystery deepens. I mostly have RDS-B because of the dividend tax. I read from reputable source about the dividend tax being phased out Jan '20 so I started a small position in RDS-A after Jan '20.
Well today I got the dividend and RDS-A dividend is still 15% less than RDS-B. Tomorrow I am going to convert all my A shares into B shares, no big deal for me.
I follow Brexit since the 2016 referendum because my in-laws are British and we go there almost every year so I check the pound/dollar rate daily as a matter of habit. Even though the pound loses a lot vs the euro, especially the pound/euro exchange swinged a lot during Theresa May tenure to push for the Brexit vote in parliament, the difference between A and B shares stayed the same and both types of shares moved in unison with oil price. So the ADS seems to be immune to currency exchange rate between pound/euro. Your theory is about the best explanation there is for now.