Mysterious case of Insufficient Storage despite 5.08 GB of available space.

I'm using the Nobis NB09 tablet with Android 4.1.1. When attempting to download an app from the Google Play Store, I get Insufficient Storage error, despite 5.08 GB of available space and the app that I want to download is only 9.83 MB, so there is more than enough space to install it.

Available space reported in Storage:

Internal Storage

Available

66.24 MB

Internal Storage

Available

5.01 GB

Why is all this available space being ignored?
Why are there 2 sections for Internal Storage shown in Storage?

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