Both mean same thing : like in Asymmetrical IUGR which can be due to fetal hypoxia /anemia / acidosis : there’s brain sparing effect which means other body parts will receive less blood than brain . So you will get diastolic flow in MCA but not in aorta
The middle cerebral is a high resistance vessel. In normal situations, flow is minimal. In case of anemia, there will be hypoxia. ( same situation as in IUGR. ) This redistributes blood in the brain, decreasing resistance in MCA, so there is increased flow of blood.