I have to agree with Mrs Riva. I don't perform nearly as well on the treadmill and it's because I'm not able to maintain my own pace. I need to slow a bit here, speed up a bit here, but it's all done without thinking, and it's all done at my own timing. Just because you keep up an 8:15 mile doesn't mean that you are moving the same speed the whole time. You fluctuate minutely all the time based on feel.
When you are on a treadmill, it keeps the same pace constantly, there is no variation. You aren't going at your own pace, you are keeping up with whatever the machine tells you to do.