Why do some volume controls start at -80db or..?

On level scales that indicate zero full open, '0' is NOT no gain, it's FULL gain...as much as the device was designed to create. The minus portion of the scale is that number of deciBels BELOW FULL GAIN.

Such a scale is quite useful in displaying just how much of the Voltage gain built into our preamps/amps/receivers that we DON'T use.
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The reason for the differance between the two is the Krell is for 2 channel and the Yamaha is for HT. I think as I do not have manuals on your equipment. Your Yamaha has a calabrate mode so you can set the volume, distance and envelope of your speakers. You calabrate your speakers so all of them will be the same at zero volume. The zero setting is really between 75-85db for home theater. When the volume is set to zero you are at the recommended volume for movies. It is meant as a referance point. Your Krell I do not think has a calabrate mode.
I think 75db is recommended for home theater and 85 db for commercial theaters.
IMHO

This has been a really useful thread. My HT amp goes from -80 to 0 and I have asked this question elsewhere and have always come up with a queer look of puzzlement from my would-be respondents. Thank you all for the insight.

Interesting responses, most times the numbers on a volume control or attenuater mean little. Keep in mind, attenuate simple means a reduction of amplitude of oscillation, so removing the volume control from the circuit would give you full power.

BTW my Meridian G68 can display volume as -86 to +12,1 to 99 & no display.