It's beyond me how someone can believe you can carpet bomb a city where 2 million people live and kill fewer than 40,000 people. In fact, I don't find myself able to believe that people genuinely think that.
You're comparing a) the number of named, positively identified deceased people counted during an active conflict [1] where Gaza's healthcare system has been targeted on a daily basis with b) the final body count [2] from something that took place 79 years ago and has been studied by historians for decades. They're not comparable. In actual fact, fatality counts of Palestinians have been subjected to a standard far more stringent than anything applied during other ongoing conflicts, at least in "western" media.
It will probably be a decade before anyone is able to dig up the mass graves, sort through the rubble, interview families (that haven't been entirely wiped out), and come to a more complete list, and even then there will be a large number of unidentified deaths and the estimate will be a range.
[1] Really the word "conflict" is inappropriate here, because most of what's taking place is attacks on Palestinian civilian. The actual military conflict appears to be only a small percentage of the overall violence.
[2] Incidentally, both the high and low estimates for Hiroshima deaths also include people who died from the after-effects of the bombing, not just people who died during the attack itself.