A whole bunch of things can effect your performance on any given day besides how fit you are. Weather conditions, the terrain, how well you've warmed up, your capacity to relax under greater stress, learning to pace yourself and adapt as conditions change, your ability to run your own race and not be influenced by those around you.
So if you enter a race do your homework first, check out the terrain, look at the weather forcast, make sure your diet is tried and tested. If you look after all the small details then youll be able to set a realistic target for yourself on race day, running with confidence is a huge advantage and a smarter way to run.
What you should be looking for is an average speed over time not a rigid time per mile and even then you need to be flexible depending on the conditions.