It doesn't matter whether the wood is kiln dried or air dried. The problem is that it changes with seasons and wood moves as it gives up, or takes in moisture. It gets wider across the grain when it takes up moisture (typically in the summer when indoor relative humidities are higher) and it gets narrower when it dries out (typically in the winter because indoor RH values are lower). But it only gets longer/shorter ACROSS the grain (length along the grain doesn't appreciably change), which is why the end pieces are now longer than the width of the table. OP needs to change design.
I would recommend the OP google "humidity, wood movement and furniture design" before building any more furniture. After reading and understanding that, maybe venture into "bread board table design."
In the near term, I like Andrew's suggestion for a kludge as much as anything else. Short of redesigning and rebuilding, there is no way to "fix" this table. The best one can do is shmutz a pretty face on it, plead ignorance, beg forgiveness and hope for the best. I don't think anyone who ordered such a table based on the Pinterest photo was expecting Fine Furniture. It is what it is, and caulk (with maybe some foam carpet padding behind it) may be the OP's best friend. Good luck.