Arduino Uno R3 ATmega328P Extended Fuse Byte

Just purchased an Arduino Uno R3 and programmed directly via ISP into the ATmega328p. In the process I erased the bootloader. Great learning experience, however. Just a question about the fuse bits. According to the boards.txt file, the uno.bootloader.extended_fuses=0x05. Looking at the ATmega328p data sheet_10/2014 p.282, the extended byte is 0xFF by default, with only the last 3 bits BODLEVEL2/1/0 as programmable. On p.305 they have a table giving 101/110/100 as possible values. I guess writing 0x05 will give 0xFD on a read. Any comments?

The non-existent bits may read back as 0 or 1.
0xFD = 1111 1101, 0x05 = 0000 0101, so same thing.
In the IDE: Uploading Using Programmer does not change the Fuse Bits, only Burn Bootloader changes the fuse bits per the settings in boards.txt.