This statistic shows average utilization across all disks. The per-disk breakdown shows the utilization that that disk contributed to the total average, not the utilization of that disk.
This statistic may be useful to trigger an alert based on the average for all disks.
Investigating disk utilization is usually much more effective using the standard Analytics statistic Disk: Disks broken down by percent utilization - which instead of averaging utilization, presents it as a heat map. This allows individual disk utilization to be examined.
The disk breakdown shows the contribution to the average percent which each disk made.
A system with 100 disks would never show more than 1 for any disk breakdown, unless that disk was selected and displayed separately as a raw statistic. Such a system would also show 0 percent utilization for disks less than 50% busy, due to rounding. Since this may be a source of confusion, and that there is a better statistic available for most situations (Disk: Disks), this statistic has been placed in the Advanced category.
See Disk: Disks broken down by percent utilization for a different and usually more effective way to display this data.