Why is it?

Why is it that when you have a certain time to get dinner done and you're cooking something you have made before and it always takes you 45 minutes to cook that it takes longer then usual? A friend of mine at church just had a baby so I was making dinner for them tonight. I made chicken and rice that I cook with cream of mushroom soup. It always takes me 45 minutes to cook and it's perfect. Tonight, it took over an hour so I was late getting it to her. She said it was ok because they are late eaters and don't usually eat until 7pm so it was fine that I brought it at 6:45pm. But it's still frustrating that it took so long. I hope that it tastes good though. If it doesn't, I also gave whole wheat rolls, steamed veggies, sliced oranges and a banana cream pie so if the chicken tastes bad, at least there's other things I made that will be good. :)