Is phosphorus a stereogenic centre in this phosphate ester?

My textbook says that in the above diagram that there is only 2 stereoisomers. I am guessing this is due to the stereogenic carbon atom and not due to the phosphorus atom which apparently isn't a stereoisomer.

However, it states that the above molecule has 4 stereoisomers. This must indicate that in this case the phosphorus atom, it is a stereogenic and gives rises to 2 stereoisomers.

Why is the phosphorus atom in A not a stereocenter while the phosphorus atom in B is a stereocenter?

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• An even better answer would point out that there isn’t really a $\ce{P=O}$ double bond in that molecule and that $\ce{\overset{-}{O}-\overset{+}{P}R2-\overset{-}{O}}$ would be the better depiction ;) – Jan Jan 7 '17 at 19:06