Epistemic status: I think this is right but it's surprisingly hard to find clarification on this so I could be missing something
Positive-sum interactions means the total gains outweigh the total losses - i.e., the pie gets bigger.
A win-win situation means both parties come out better off - they both have more pie than they did before.
These terms are often used interchangeably but by these definitions: while a win-win must be positive-sum, not all positive-sum interactions are win-win.
For example, if we both start off with $10, and through some interaction I end up with $20 and you end up with $5, we've increased the total pie size from $20 to $25 (positive-sum) but you've ended up with less than you had in the first place (not win-win).
Using these terms more precisely seems like it could avoid troublesome miscommunications, especially when discussing things like the possible impacts of new policies. I expect most people would choose a slightly larger pie where no one loses out (i.e. a Pareto improvement) over a much larger pie where some people end up worse off - i.e., whether something is win-win is likely more pertinent than whether it is positive-sum.