Both numbers are "accurate", but one is instantaneous and the other is an average over time. When you launch AirRadar the initial set of readings you see should show Signal, Signal Avg, and Signal Max should be the same because there is only one reading to average. After that it could go either way. On my network all three readings are often the same or are within a point or two either way of one another. For example, at the moment from my iMac in the same room as the Time Capsule that is generating the network I am seeing
Code:
SSID                 Signal         Signal Avg         Signal Max       Noise         Channel

Bailey                54%               60%                   76%              0                   6
Bailey (5GHz)    63%               64%                   65%              0                   149
Bailey (5GHz)    15%               16%                   18%              0                   149
Guest Network  59%               59%                   74%              0                   6


The second "Bailey (5GHz)" is the signal from the Airport Express repeater at the other end of the house.

Niño is obviously encouraging you to return your repeater to the store and although the firmware update seems reasonably doable — I would take his suggestion and get a new repeater.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein