Hello PitotTube,
Without pretending to be an electrical expert here (i chose to be a pilot instead) i'll give awnsering your question a go.
A low amperage, as i'm sure u know, indicates that there is little electrical load. As you can read in the Boeing manual: "Under normal conditions, DC bus 1 and DC bus 2 are connected in parallel via the TR3 disconnect relay. In this condition, TR1 and TR2 are each powering DC bus 1 and DC bus 2. TR3 powers the batterybus and serves as a backup power source for TR1 and TR2"
As you see, TR3 doesn't really come into play in a normal situation. The reason why the current indicates half of the TR1 and TR2 values i'm afraid i am unable to awnser. Maybe someone more proficient knows?
Hope to have helped you a bit..
Per ardua ad astra