Today, serviced a dining room table. The client said that her table seemed like it was sagging when the leaf was in. Well, when I got there, we put the leaf in and what I saw was the top had warped (bowed) slightly dipping down towards the ends (creating the table and leaf to be unlevel). I moved the alignment clips under the table (the 2 outer ones) closer to the ends. That fixed the unlevel problem. But she still had the slight bowing problem. Noticeable and not noticeable with a tablecloth down with all the settings. Depends on your tolerance. Anyway, this bowing issue was caused by the manufacturer did NOT apply any sealer or finish...yup, raw wood - veneer. There's a term called "balanced construction". What you do to one side, you have to do to the other. So if there is raw exposed material on one side and 2 sealer coats + 2 topcoats (which is typical), air can reach the wood on 1 face (surface), 1 the air cannot. This creates warping...in this case, bowing. Nothing usually can be done in this situation because the runners underneath get in the way for braces or cleats to straighten the warp.
I know this was long and detailed, but if you can relate...now you understand. And now you know.
Cheers!
Dave
Sorry for not having pictures to illustrate...getting older I guess.