sounds like you're playing with a 8D car, read the BINs out as 27C256 chips. read it out and check location 0008, should be 8D.

the checksum will normally be updated by tunerpro when saving the file, so if it's different it's not a problem since there have been changes to the chip.

checksums are a fast and efficient way of determining if the ECM can read all of the bytes of a PROM correctly. it starts adding up all of the bytes after a certain location (8007 in the case of 8D) and goes until the end of the PROM at FFFF. then it compares the sum against the value found in the checksum location, 8006-8007 for 8D, if it matches, the ECM figures the PROM is good, if not it goes into limp-home mode. if the mask ID is changed to AA, the checksum is ignored, generally this is done during GM's engineering phase or while emulating.