Basically, your wishlist (and maybe other pages as well?) display the sale % using the wrong number. It makes it look like you're getting a better deal than you are. For example, this game I wishlisted is $43.95 AUD. But if I get it in the bundle, it's $41.81 AUD. That's a 4.8% discount. But the wishlist shows 42% discount of the whole bundle, not the actual game I wishlisted. I think that's misleading.
You could argue I'm getting extra games so it's a good deal; but I don't know that other game, I haven't researched it or wishlisted it. In some cases, I've had this happen to games that are in bundles where I already own the other games. At that point I'm just getting completely ripped off, because it says 75%, but it's actually 5%, without even any "bonus games" to justify it.
To avoid this, never buy from the wishlist or home page or any page that isn't the original full store listing. I always click into each game to see the original store page and ensure it's a real discount before buying. I'd recommend doing the same.
I noticed this years ago and submitted a support ticket and a Reddit post, but it never got fixed. I feel like it must be by design at this point. This is the first "anti consumer" practice I've seen Steam implement.
Oh. Yes. I found that confusing, too.
I think the discount displayed is always off of full price? So it might actually not be a discount for me, at all, compared to one of the bundles I could complete.
I do like that it grabs me the best price - when I put it in my cart - but it has surprised me when the bundle ended up in my cart.