checksum is not correct (125Khz RFID module RDM6300 - UART)


I have some problems with the checksum for my RFID project. I’m using the ‘125Khz RFID module RDM6300 - UART’ and an 'Arduino Pro Mini 328 - 5V/16MHz’ + ‘RFID tag combo (125khz) - 5 pcs
When I’m reading RFID tags, I get for example the following tags and checksums:

0400B08EFD <5
3C00CF030F @@
03007CD382 KA
52006C0870 4I
52006D6980 =O

My problem is, that the checksum is neither a hex-value nor correct. If I’m right the checksums should be:

0400B08EFD C7
3C00CF030F FF
03007CD382 2E
52006C0870 46
52006D6980 D6

Has anyone any idea what could be wrong?
Thank you for your help!!!