Apparently, 275gb memory is not enough to load 3 studio sets which take up 12gb. Why?

I have 64 gb of Ram, 275gb of spare memory in my imac, and yet I still get Logic's 'Not enough free storage space to install selected instruments and loops' message, telling me to delete files and empty my bin (already empty).

This is for downloading three new studio sets for Logic 11. Piano, Bass and electronic drums. This is supposed to require, 11.64 gb, so it tells me. Why does my computer think I have less available memory than I actually have?

By the way. The same thing happens if I try to load them one by one. I have seen this offered as a solution but it doesn't work for me.

It's very frustrating. Can anyone help?