I'll give you the benefit of the doubt and assume that you are not just deliberately doing the old "just asking questions" thing and are legitimately having difficulties with understanding relatively simple concepts.

There is a difference between their internal numbers and their outside communication. It's quite easy to keep an accurate head count during peace time and they did that, for taxing purposes alone - and to maintain the level of control that they wanted as the rulers of an oppressive police state. Just in case you didn't know what kind of place Gaza was before the war.

However, they actually have no idea right now, because it's war, it's chaotic, with shifting front lines, people being displaced, caught under the rubble, disfigured bodies with no identifying marks, families getting torn apart and not knowing what happened to their loved ones, etc.

You know, the same chaos as in every other war. You can't keep accurate records under these conditions, especially considering that everything even remotely resembling an official Hamas government office has been bombed to smithereens or overrun by ground troops - but Hamas pretends that they do, with precise casualty figures published incredibly frequently. That's not how this works. Israel needed months for an accurate count of their deaths from just October 7th, despite the fact that the fighting within Israel lasted only for a day or two. Ukraine has huge "unknowns" about their population in occupied areas and where there has been or is fighting.

Yet Hamas of all people somehow are omniscient record keepers - and barely anyone is willing to question it. We then get a game of telephone of increasingly reputable parties repeating Hamas numbers to one another, until people can naively say that "the BBC" or "the UN" is reporting something, oblivious to the actual source of the information. It's as if nobody learned how to properly trace sources at school or university.