Until now we could not figure it out. Then, Josh came across this tip from Paula Shoyer in our The Kosher Baker cookbook.
And it all makes perfect sense(especially for Josh who is really excited because it connects perfectly to one of his science lessons). Whenever I bake, I gravitate to our old pans, ones that were my grandmothers that I"I prefer the lighter-colored stainless steel or aluminum cookie sheets. Cookies baked on the darker, often non-stick, cookie sheets will bake faster because the darker sheets retain heat better. If you are using a dark sheet, check the cookies one or two minutes earlier."
Josh likes to use the nice shiny, new, pretty pans we got for our wedding.
Now we know, when using the old pans, follow the recipe. When using the new pans, bake for 2 minutes less than the recipe calls for. Such a good tip!