Why might historians disagree about the number of people who died because of the Black Death in the Medieval period?

Historians do indeed disagree over the precise number of fatalities caused by the Black Death. Estimates vary as to the number of people killed by this deadly pandemic. Some say 75 million died; others, 200 million. Though historians may disagree over the numbers, there is unanimous agreement over the utter devastation that the Black Death wrought upon the continent of Europe.
The huge discrepancy in the overall number of deaths between different accounts is largely due to the lack of reliable records. While some parishes were able to keep fairly accurate records of plague deaths, others were not. Generally speaking, large urban centers such as towns and cities were more reliable at keeping records than rural areas. One should also bear in mind that, because of the plague, people naturally had other things on their minds than keeping precise records of who had died. The ensuing gaps in the records have only been partially filled by extensive historical research over the years, still leaving much room for guesswork. Irrespective of the precise number of fatalities, though, there's no doubt that the Black Death wiped out a substantial proportion of Europe's population, earning it the reputation of one of the deadliest pandemics in history.