I have a discount that works how I want it to with a flat rate but I need to switch it to percentage. When I do that the same options start behaving differently?

I have it set to the following. This seems to work how I need it for flat rate...

Customer adds one item to cart, they get £1 off. If they add 2 items to cart, they only get the £1 off the first item. If they add 4 different items, they get £4 off. If they add 2 of one item and 2 of another they get £2 off.

I want to do the same with a % so customer adds one to cart, they get 10% off. If they add 2 to cart, they still get a discount but only 10% off the 1st of each item. I've got exactly the same settings, just changed it from flat rate to % but now for some reason the % is applying to all items, not just the 1st one. What do I need to change?


Discount Amount:
1
Scope:
Applies storewide but to individual products