Android storage , where did my 2GB go?

The internal SD-card of the device is probably partitioned.

Check out this screen in the app you mention in OP:

Since my SD-card is not partitioned, I only see one partition. In your case, you should see more than one. The combined sum of all the partitions should equal the total size of the SD-card.

You could root the device and attempt to change the partition table and merge the partitions. It may be a bit risky, though. But it's certainly worth trying if you're open to the risk of rooting, tinkering, etc.

You may also try apps such as this one to move apps to the storage partition. You can free up space on the app partition that way.

If you want to root, you can also use this app to remove bloatware system apps that you don't need. This can also free up space. However, this may also be risky. If you try this, you should be prepared that something might potentially go wrong, and you might have to re-flash stock firmware. That said, removing preloaded bloatware is often easy and trouble-free.