That is a great question but as far as I am aware of the answer is that we don´t know. In a classic paper by Riggs it is stated that:

"The elaborate embryological evolution of the circle of Willis and the shifting patterns which precede its definitive form predispose to abnormal development"

and that:

"the investigation of anomalous formation of the cerebral arterial complex has been concerned chiefly with defective development of individual stems and the relation of such abnormalities to embryologic or phylogenetic evolution."

All we know for sure is that a "normal/complete" Circle of Willis is only present in about 50% of the general population with little geographical variation

Thank you for your response. Do you by any chance have the name of the paper by Riggs?

Sure. Happy to help. This is the citation:

RIGGS HE, RUPP C. Variation in Form of Circle of Willis: The Relation of the Variations to Collateral Circulation: Anatomic Analysis. Arch Neurol. 1963;8(1):8. doi:10.1001/archneur.1963.00460010024002.