That's partly because the drag effect of the atmosphere depends on the solar activity. Meaning, depending solar activity an object will start aerobraking higher

That's partly because the drag effect of the atmosphere depends on the solar activity. Meaning, depending solar activity an object will start aerobraking higher up or lower down in the atmosphere. That of course drastically impacts the time it takes the object to fall out of orbit and since we can't really predict solar activity we cannot precisely predict when nor where this battery pack will hit the ground.