A company called 'Microsoft' promulgated a specification for the 'Simple Boot Flag':
Bit 0 indicates that the system has a PNP OS ; Bit 2 indicates that it should run diagnostics at boot time. The BIOS 'owns' the SBF at boot time, so it gets to set it. I assume Linux doesn't mind being identified as a PNP OS, so it must object to being told to run diagnostics.
Usually an OS would clear this bit after it had run diagnostics. Either Linux doesn't clear it or the BIOS keeps on setting it.