This question has been addressed by Sri Jiva Goswami in his treatise Tattva-sandarbha. The Puranas were written with different people in mind. Some Puranas were written for those worshiping in the mode of ignorance, some for those worshiping in the mode of passion, and some for those worshiping in the mode of goodness. As such, each Purana extols the manifestation of Godhead that best suits those for whom the Purana was written, and even minimizes the position of higher manifestations of God described in other Puranas. The idea is to encourage people to worship by allowing them to conceive of their Deity as supreme. Everyone has to start somewhere.
