Theory #1 - The wife likes to make her husband bend over a lot, because he has a prosthetic pelvis that is subject to a manufacturer recall and is due for replacement. This is funny.
Theory #2 - The man needs to lose a little weight, so his wife chose this moment to exaggerate his width, by acting as though she hit the ball "his way", as if he were twenty feet wide. This is funny.
Theory #4 - Hoping to add a little spice to their domestic life, the couple has agreed to play ping pong to decide who will extract information from the prisoners this evening. This is funny.
Theory #3 - That's not a ping pong ball. This is funny.
Jim D. has found a clue! Theory #4, coming your way! - Lamp is set low enough to interfere with husband's vision, high enough for wife's line of sight to be clear. Wife's continuous pretense that she can't see exactly where husband is standing bamboozles husband into thinking playing field is level. Loser subject to extreme sexual humiliation. Thus, wife's statement is sarcastic. Sarcasm is funny.
You see, back in them days, readers was willing to work a little extra harder to "get" the joke. Not like us modern-age devolved folk, who are so dependent on instant gratification that we even resort to watching SpongeBob episodes speeded up on YouTube!
Theory #4 comes to us from "anonymous", doing a very poor job of concealing his identity. - In an attempt to retrieve the ping pong ball, the Wife expels gas in the Husband's direction & feels the need to "warn him" of the impending nasal offense - in his typical fashion. This is funny, in a vengeful sense.
[Commenter theories will be added to the post. -Mgmt.]