Well we all know the scavenge pumps can cause a slight imbalance, but this is fuel fed from the center tank and not added.
Correct, but it could have been fed into the CTR TANK from #2 TANK, inadvertently, for mechanical reasons, a slow rate. Then, being suction fed into the #1 TANK it materialises as a slow increase in #1 Fuel as the CTR TANK is scavenger empty.
As the rate of scavenge exceeds the rate of fuel flow into the tank (which to cause and imbalance must come from #2 TANK) this shows up in such manner.
Thus I stand by the (no unlikely) position of INADVERTENT TRANSFER OF FUEL INTO CENTER TANK possibility, from TANK #2.