The MBR and the boot sector of the boot partition are two different places where it is common to install stage one of Grub.
If stage one of Grub is installed in the MBR, I don't think the boot sector of the boot partition would be used at all.
If stage one of Grub is installed in the boot sector of the boot partition, then some other boot loader must chain load it. The BIOS does not load the boot sector of a partition.
