I'd go with the license usage tab to understand the license count.
On the borrow tab, an alternate logic was applied. Because a Premium or Pro will always take a Standard with it when you borrow them, we subtract a Standard license for each of them on the borrow tab. They are, in a way, being reserved in case you want to borrow a higher package. In your example, there are actually 5 free Standard free, but you also have some combination of Pro/Premium in excess of 5 free.
The display of free licenses on the borrow tab is confusing and something we are looking to address in a future version.
Thanks for clearing that up for us!