re: Refinish for table

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?).

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?

The last comment made me laugh. Since when we leave the house our dog does go on crumb patrol. 
Seeing that closer. Like splint said it might be from perspiration or oils from skin contact.