[nsa_sailor]

I think it's because there are only 5 discrete levels for the Ring volume.
You can verify that yourself by going to the Settings > Sounds screen and sliding both the Ring and System volumes. The System increments by 6.25% each (16 levels) and the Ring increments by 20% (5 levels).
I did notice that if you set the device to use the single volume setting, you can get 6% increments, but I'll wager that in actuality, the ring volume gets rounded to the 20% increment either way.