It's just not the same day
Some days are better than others. A couple of days ago, I ran the fastest I probably ever have for that distance. I was so happy and proud. But today, I just couldn't keep up; my pace was as slow as it was a couple of months ago. I kept having pain and couldn't breathe because my nose was dripping.
There are so many factors in a day—some controllable, some not. But it's okay, because what matters is that at least I got up and ran. :)