one thing i noticed is that in som versions of the boards.txt differs slightly.
###############################################################
attiny13.name=ATtiny13 @ 9.6MHz (interne 9.6 MHz clock)
attiny13.upload.using=arduino:arduinoisp
attiny13.bootloader.low_fuses=0x7a
attiny13.bootloader.high_fuses=0xff
attiny13.upload.maximum_size=1024
attiny13.build.mcu=attiny13
attiny13.build.f_cpu=1200000 in some versions of this its 9600000 or 100000L
attiny13.build.core=core13
###############################################################
I have understand that this is the setup of the internal clockspeed, nothing else.