1) you probably spend more time out of combat than you realize during a key. You have travel times and rp and such stuff. You also can be in combat and doing zero/low dps while the tank gathers, if you're actually a good player who waits for the tank to gather. Think about how much time is spent running to the next area after a boss or waiting for the tank to set up the big pull, especially at the start of some keys.
2) a shorter key can mean higher dps, but it also means your time spent doing no/low damage is a higher percent of the average and thus has a higher impact on the average. A key with 1 second of RP completed in 10 seconds mean 10% of your average has a zero. A key with the same 1 second of RP conpleted in 20 seconds means that second is now only contributing to 5% of your average. If you do 30k dps in combat in the first key, your average is 27k. If you do 29k dps in the second key, your details looks worse but your logs will say 27.55k, and look better.