Interesting... (great table BTW!)
That segment of the top looks to be slightly darker than the rest (from a different plank/tree?).
Being darker, it could be reacting more to perspiration or something related to the clothing (always long sleeve shirts?).
Could make it a routine to rotate the table every few months (like a mattress) to "spread the patina".
Perhaps, though no real evidence, could the culprit be lurking under the table (photo let center)???
Maybe she likes licking the table there during cleanup duties after plate cleaning and putting the dishes back up?