Center tank indication below any significant amount is always prone to error, as it can vary already during loading based on forwards strut compression. In flight, this inaccuracy remains with small amounts.
Best guess would be as already revealed by others, scavenge into the left, which would be consistent with a gain in #1 tank.
However, it cannot be ruled out that there may be some "inadvertent transfer of fuel into centre tank" (a small amount of fuel), which used to be a QRH item [now removed], when it is only a small amount, when occurring will be taken care of by the scavenge system into #1 tank.