Available storage space doesn't add up (appears to be doubled) (Nexus 9 - unrooted)

hoping someone can help.

My 32GB Nexus 9 shows 21.64GB of available storage space out of total space of 25.61GB -- seemingly 3.97GB used. However, the used space only adds to 1.97GB. So there is apparently another 2GB of non-system data being used somewhere unreported?

Strangely, My 32GB Nexus 6 reports the identical apps and data correctly - 24.07GB available out of 25.98 total.

Does anyone know why this is occurring and how to fix on the Nexus 9?

Many thanks