I find the percentage value is wrong when looking at the end to end availability for a group of devices.
For example: Server 1 - 97.5% Server 2 - 96% LAN - 98% Host 1 and 2 - 99.96%
OpManager reports the average percentage (97.87%) for the group, not 'host x LAN x Server 1 x Server 2' which is the correct formula for calculating end to end availability (which equates to 91.96%)