Why do cats do this?

Two identical plates which had been filled with the same food.

They will only eat from the left hand bowl and Saphy is waiting patiently for it to be refilled.  If I refill it by switching the two bowls over, she will then eat it.

J says they are genetically predisposed to only eat from one side – this does not sound plausible to me but I can’t think of any other explanation except that they are contrary animals.  Wait – that will be the reason..