A straight forward answer and a puzzling reaction

Every once in a while I will be engaged in a discussion regarding the expected life span of my work.  This seems to come in two forms and each form has distinctly different reactions. Some will, upon inspection, note that they are certain whatever they are looking at will last for a very long time. It is something that they appreciate in the piece.There is another group of people though who ask how long I expect something I built to last (and thus how long they should expect it to last). Their reaction is different and I'm not exactly sure what to make of it. Talking with a potential customer a few weeks ago provides a perfect example. While at the Art Fair in Loveland earlier this year a man walked in and inquired about one of my humidors. This humidor as a matter of fact.

Mahogany and Planetree burl humidor

We talked about the solid Spanish Cedar lining. We talked about the interior size. We talked about the humidistat. Then he asked me how long I thought the box would last. I answered, "You won't live to see that day. Chances are, neither will your children. They will be leaving it in their will to one of your grandchildren." I went on to say that with normal use and care 100 years should be within easy reach and longer being quite possible. When I said this he got this mostly blank and slightly bewildered look on his face. He isn't the first. In fact, some variation of that response seems to be the general rule. Sometimes it is accompanied by them saying "Oh...." which kind of trails off. You would think that I started breaking out physics calculations involved in long distance sniper shots or something (well, once you account for spin drift and the movement of the earth while the bullet is in the air using these formulas...). My first question is what reaction am I actually seeing? My second question is why?