Weight Rate not working

Testing the marketpress on this site, I set up a weight rate of 1 kg the price is 10 (DKK) and 2 kg the price set to 20

I made a product at 0.4 kg
on at 5 kg

When I put one product at 0.4 I get price 10 (correct)
When adding 5 kg product I get price 20 Correct.

BUT: If i add only the 0.4 and choose 10 units (4 kg) I still only get price 10. Should now be 20.

I also made variations of the 0.4 kg product. I made a 0.25 and a 0.35
If I put one of 0.4 one 0.35 an 2 of 0.25, I shold have a weight of 1.25 kg. But again it only write price 10.

Seems to me Marketpress only look at the single item price and not calculate total weight?