Could be b'cos you have a AFF-A300 which is a ALL-Flash array, and since 9.2 bydefault 'Dedupe' allows block-level sharing across volumes within an aggregate (i.e Aggregate level dedupe) which further reduces the physical used space needed to store data. So, logically 'used data' across your volumes is 16TB ; but actual space consumption on aggregate level is reduced.
Keep in mind, you have two different file systems here, WAFL at the aggregate level, and WAFL at the FlexVol level. Both of them now on AFF have the ability to do some form of storage efficiencies. Also, FlexVols can be thin or thick provisioned in the aggregate. So you have to think about what the counter is implying when you are looking at the data.