The number of different possibilities exceeds the amount of time available to make a verified account of the involved individuals and their specific circumstances. Dynamic conditions can make an accurate accounting time-dependant. By the time the information is reported by one person, it may have changed, making the existing report incorrect. Where data is in conflict, other time-sensitive information must be located and compared from an acceptable verifiable source. Multiply this situation by all the possible "hospitals" around the city and you get an idea of the complexity.
Also, who does the calling? Where does the call go? Who answers these calls? Who verifies the information? How often is the information updated?
The sheer complexity of assembling the unassailable true and stable answer makes a complete tally of the injured not possible for the convenience of the Fox News broadcast time by the police spokes-person designate is one very probable answer to why.