My guess (and it is just a guess) is that the "official" who said it could be as high as 10,000 was probably responding to the incessant questions that kept being asked by every reporter to every official in every interview...
"Could it be in the hundreds?"
"Could it be in the thousands?"
"Could it be as many as 10,000?"
After saying "We don't really know," the official probably answered, "It could be, we don't know."
Since day one, there has been a quest for a body count.
The 10,00 number came from the idiot Mayor.