I've been lifting weights for 20 years and this is what I have discovered too. You can keep your sleep/nutrition/work schedule very consistent and some days things will just be harder in the gym. On those days, I will do a little less extra exercises and just stick with the basic compound movements that I was going to do.

On the flip side, there are great days too. Take advantage of those and get a little more done in the gym. Overall it's about consistency of the practice, not about an individual session being great.