These are just descriptions of the vascular structure, not notes of defect. The radiologist lists all types of characteristics whether they are normal or abnormal. I had vascular abnormalities noted in my MRI/MRA and PET scan. They are quite common and rarely cause symptoms.

From what I can find, this likely means both arteries provide blood flow to the brain together. It can be by a crossover vasculature. This reference could be noting this crossover characteristic. When one is smaller than the other, crossover vasculature often develops to provide equal blood flow to both sides. The arterial system has unique ways of overcoming flow limitations. Heart restrictions often cause new arterioles and capillary routes that carry blood around the blockage/restriction.

If the radiologist did not list any INDICATIONS or CONCLUSIONS at the end of the report, it would mean there was not a concern.

It is important to not let irregularities cause anxiety. Everybody has irregularities on their anatomy.