The Worst Ten Minutes of the Day

Each night we make sure the girls' backpacks are ready, clothes are set out and lunches are made.  So why is it so hard to get them out of the house in the morning? 
Some days the problem is obvious.  The girls spill something on their clothes and need to change at the last moment.  The girls cannot get their shoes untied.  The girls have to go to the bathroom at the moment we are supposed to walk out the door.  The girls cannot figure out which fashion scarf to wear with their denim jackets.  You get the picture.
Other mornings run like this morning.  We were five minutes late getting out of the house and I cannot even tell you why.  The backpacks were packed.  Lunches were ready.  The girls were moving at a reasonable pace.  Still, we didn't get out of the house on time.
Now that the weather is getting colder -- and it's soon to be snowier -- we need to push our out-of-the-house time back.  We cannot leave five minutes late and make it to school on time.  Even though there's a 10 minute window during which the girls need to be at school or they are late, I don't like to cut it close.  I'm not interested in finding out whether or not the school's clock syncs with ours.  I want to be there as soon as the school doors open.
Leaving five minutes earlier means adjusting the schedule so we can leave on time. The problem is that no matter how well I plan, there are some things that are out of my control.  The ten minutes it takes us to go from "let's leave" to actually locking the front door will still be the worst ten minutes of the day.