The difference in P50 values of fetal and adult hemoglobins is best explained by a difference in the extent of subunit cooperativity
P50 is the partial pressure of oxygen (Po2) which means hemoglobin is 50% saturated with Oxygen. It measures affinity of hemoglobin with oxygen.
Hemoglobin that transports oxygen in the blood contains four polypeptide subunits which are 2 alpha chains and two beta chains. Subunit cooperativity is defined as interactions between hemoglobin subunits
The hemoglobin affinity for oxygen is more in fetal hemoglobin than adults as fetal haemoglobin subunits gamma chain in fetal instead of beta chain as in adult.
Therefore, in fetal hemoglobin P50 value is less than adult hemoglobin and less value means greater affinity.