These two FCC rules will explain why...
Section
(3) When a mobile station transmits on a different frequency than
its associated base station, the assigned call sign of either the mobile
station or the base station may be transmitted. Further, a single mobile
unit in the licensee's authorized geographic area of operation may
transmit station identification on behalf of any other operating mobile
units in the fleet.
(4) Use of an identifier other than the assigned call sign. (i) In
the Public Safety Pool, mobile units licensed to a governmental entity
and which operate on frequencies above 30 MHz may use an identifier
which contains, at a minimum, the name of the licensee if the licensee
maintains at the station a list of the special identifiers to be used by
the mobile units.