Hi,
We are sorry for the delay, it's been a long time we should have answered to this question.
So as I can see, you are not using any discounts via our menu Order > Discounts.
But based on the retail price you are displaying a reduction percentage and as the "retail price" can be set only for the main product it is causing issues when displaying a variant, the reduction become a negative one.
The solution could be to create a custom product field, fill the original price value for each variants and edit the view "product / listing_price" in order to display the correct percentage based on the custom field value.
By default we are not displaying a percentage based on the retail price, so it is something you added. But this was not based on the correct value