In the SQL query in NodeAvailabilityReport.jrxml
node_outages.avail_total is being calculated as total number of seconds in the period being included (e.g., 604800 for a 7 day period)
outage_seconds is being calculated as the sum of individual outages, not taking into account the number of interfaces in each node.
This causes the outage time (and outage percentage) to be effectively multiplied by the numeber of interfaces in a node with multiple interfaces monitored.
I saw this on 1.8.11, but looked at fisheye, and the query remains the same.