cannot update applications because of confused memory
The OS attempts to maintain a specific threshold of free space, and that's why you appear to have plenty of space to install something but you get the warning and it fails.  As the link from beeshyams says above, it may or may not be 10% of your total internal storage. 

How do you get around it?  Utilize external storage if you are able, or any other strategy you can use to free up the internal storage space.