Independent Samples

I cannot understand why the Area Under the Curve (AUC) is not the same when I calculate the AUC for each of the Groups and when I then compare the two groups.

1) Calculation of AUC, for each of the groups (SPSS: Analyze/Classify/ROC Curve):

Group A ("10"): 3223 observations | AUC=0.752

Group B ("20"): 1834 observations | AUC=0.695

2) Compare two Global Data Groups (SPSS: Analyze/Classify/ROC Analysis)

Global Dada: 5057 observations with Grouping Variable ("10/20")

AUC Group A ("10"): 0.741

AUC Group B ("20"): 0.698

Why do I have different AUC's depending on the statistical test I run in SPSS? I've tried in other software and the difference always occurs. Shouldn't the AUC's be exactly the same, doing the individual calculation or comparing 2 samples?