Jasenovac concentration camp
These estimates are very different for many reasons.  The Ustaše did not keep accurate records.  Different people use different ways of estimating deaths.  Sometimes, the people making estimates have political biases.  In some cases, entire families were killed at the camp, leaving no one to submit their names to lists of the dead.  On the other hand, the lists sometimes include the names of people who died in other places; people who survived; or people who are on more than one list.