If i had to make a guess, i would bet that the oil is the culprit,its drying on the top because of exposure to air movement, at a much faster rate that the bottom, have run into this before, always be sure you use good drying oils, and simply put the pure ones aint it,if you remove the top flip it over and reverse the situation, you will probably see it reverse itsself to flat, many years ago when i had to do refinishing to eat, we used to get a ton of table leaves that were cupped and we would simply wet the concave side and then put it in the sun with the convex side up and if you wasnt careful it would reverse,cupping in many cases is simply caused due to moisture imbalance from one side to the other, and i really think this is the problem here....best of luck