Windows keyboard does not allow imac to automatically reduce brightness

[mattg3]

While recharging my wireless keyboard batteries i plugged in a dell wired usb widows keyboard.Everything worked fine except i noticed the screen was not automatically going dull and to sleep when imac not in use.Later when i disconnected and went back to wireless keyboard it went back to its old automatic sleep schedule.Has anyone ever noticed this peculiarity?Could anyone explain it? thanks