calculation of shipping costs with price setting

Setting the shipping costs I see the following issue:

for example: set: shipping cost 10 € for a purchase below 30 €/ 5 € for a purchase above 30 €

It works fine, if there is only one item with less than 30 € inside.

If you have several items like 5 x 6.30 € = 31.50 € or one item 5 times, you will still find the shipping costs for "below 30€" and not for "above 30 €"

Is it a bug or a problem of understanding on my side?

Thanks in advance