currency rate different in product than discount - using presentment currency and draft order

[Derek_Morin]

If i create a draft order like line_items: [ {product_id:1, price:31usd, discount:31usd}] in dollars (store currency). Why the draft order in presentment currency shows different amounts on the checkout page => { product:1 , price:28eur, discount: 27.50eur} and how i fix it to do what i expect it to do