Hi,
Looking at your settings, it's a lot more complex.
First, the taxes come into play. And based on how you set the "apply discounts" and "apply coupons" settings in the HikaShop configuration, it can change the results a lot.
I can see that you also have the "floating tax prices" setting activated. Combined with discounts, coupons, and the fact that the coupon should override the discounts, I'm not surprised that you're getting strange results. I don't think that anyone ever tested all of that combined together.
I think you want to set the same tax category as your products in the tax category setting of your discounts.
Also, you want to activate the automatic taxes settings of your coupons.
Then, I would recommend to try out switching the apply settings and test the several combinations.
That might already be enough.