Why? It meets their specifications.
BMW specifications are that the speedo read no lower than actual speed and no higher then 10%+2.4 MPH. So at 60 MPH, it is perfectly acceptable for the speedo to read 68.4 MPH.
'95 Alpine M3 LTW
'00 Dakar M Roadster
'02 Topaz M3
Red/White SRF #4 (Chassis 561)