Its like clothes when fading in sunlight: Colored clothes become lighter, white clothes become darker.
Seriously, with cookies, all you have to do is adjust your browser settings to ----- OOooups! Wrong kind of cookies, we try again:
With cookies, its a question of water content and sugar content.
You have basically four types of bread (cookies are a sort of bread, after all):
1) Dry, unsweetened bread, e.g. water cracker bisquits. It is dry and crisp and basically stays that way except in very humid condition where it looses some crispness.
2) Soft unsweetened bread, e.g ordinary white bread: It is soft because of its water content, so exposed to air, it dries and becomes hard.
3) Crisp, sweet bread, e.g.: many types of cookies: It is crisp because of a low water content, but exposed to air, the sugar content causes it to absorb moisture from the air, so it gets soft.
4) Soft, sweet bread, e.g.: English christmas cake: Is both moist and sweet, and basically stays that way except in very dry conditions where it will become dryer and harder.
There is a fifth type, which is the type of buns my daughter made when she was younger and less apt at bakery: They started hard as concrete and heavy as lead, and with time became worse regardless of humidity conditions. The receipe is kept secret as a service to mankind.
Hans