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?
In compound A, the negative and double bonded oxygens bound to the phosphorus are equivalent:

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In compound B, they are not equivalent:

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