In the past week, 64 Afghans have been killed in the largest bomb to have exploded in Kabul in 15 years. Source1
I presume that this sentence is in terms of grammar OK. But I tend to switch the two tenses in this way: In the past week, 64 Afghans were killed in the largest bomb that has exploded in Kabul in 15 years. The bomb attack was the strongest in fifteen years (so the usage of present perfect) and at the beginning of the sentence is the particular time mentioned which indicates that the present perfect is not permitted (so the usage of simple past). Can you explain to me the tense usage in the above sentence?