Running too slow is the only answer...!!!!
Perhaps your mind wasn`t in the run, or you were having an off day, we all have them, so don`t worry.
You are learning from experience that somedays the body feels weak and you run slower, or your mind is elshwere engaged and you run slower.
You`ll get faster again, and then woner how you can get 10 mins faster than you`ve ever run. :)
Gromit