Coupon for 15% gives weird inaccurate discounts!

Has anyone else seen this happen?

I created a coupon for 15% off all products on my ShopperPress site.

I noticed something was amiss when my customer ordered and paid me less than the correct amount.

I went in to check and did some test applications of the coupon on some products and the discounts applied ranged from 13% to 15.46%

Screenshots attached show 2 of these checkout screens for the same product, one for my customer and one from my own testing. Both discounts were incorrect!

Is there something wrong with the coupon function?