Why should a single week be the relevant metric for this?
Because using the most recent week is better for understanding how recent adjustments in policy are affecting the disease.
Although Belgium has the highest death rate averaged across the entire infection, they have implemented a lock down. Using the most recent week's deaths, we see that lock down has reduced the death rate.
In Sweden's case, they have not implemented a lock down. Accordingly, a comparison of death rates over the past week shows that Sweden's death rate now exceeds Belgium's.
By the way, here's another explanation of why Belgium's numbers are so high (from BCC online):
Belgian officials say they are counting in a way that no other country in the world is currently doing: counting deaths in hospitals and care homes, but including deaths in care homes that are suspected, not confirmed, as Covid-19 cases.