The center tank scavenge pump doesn't operate until there is less than 2 tons fuel in the left main tank.
On your first leg, you landed with 4 tons - hence remaining fuel in the center tank had not yet been pumped over to the left main.
On the next leg, the scavenge pump started to operate as left main tank fuel level went below 2 tons. So the 1-200 kg remaining fuel was pumped over.
Would that fit with what you experienced?
In some aircraft, the center pumps low pressure lights start flickering quite early. On the previous flights (where you had fuel in the center), did you wait for the master caution before switching the pumps off? Or did you switch them of at the first flickering?
To avoid (albeit small) imbalance, I usually wait with switching the center pumps off, until the low pressure light is fairly continuously lit or the master caution comes on.