Quite often this happens because the aircraft is shut down after the transfer valaves have opened during the previous flight. The REFUELG switch on the fuel panel is what resets them, so if power is removed without a refuelling operation taking place, the valves will stay open. As microburst2002 suggests, the valves are not powered when battery refuelling. When refuelling with the valves open, the fuel goes into the outer tanks and straight into the inners, without filling up the outers first.
The reason you might see outer tanks half full in this situation is if you put AC power onto the aeroplane during the refuelling process, which shuts the valves and starts filling the outers. If there is less than 1500kg to refuel from this point, the refuelling operation will end before the outers are completely full.