Hi,
Both api work identicaly, these values are only filled if you set value  with  
>vserver modify -vserver <vserver>  -aggr-list <aggr1>, <aggr2>
By default all aggregates are available by all vserver when -aggr-list is emply (-)
If you want list off aggregates just use 
 
<?xml version="1.0" encoding="UTF-8"?>
<netapp xmlns="http://www.netapp.com/filer/admin" version="1.31">
<aggr-get-iter>
  <desired-attributes> 
  <aggr-attributes>
  <aggregate-name></aggregate-name>
  </aggr-attributes>
  </desired-attributes>
</aggr-get-iter>
</netapp>
 
François