Bad days in the saddle

Today was pretty windy in North West England but i don't feel that was the reason for my bad day on my bike. As i mentioned in my thread about marmalade and butter elsewhere on these boards i thought that yesterday i had found the perfect fuel for cycling as i felt so strong and free on the bike when riding home but today after eating the same foods at the same time i really struggled as i felt stiff and weak no matter what pace i rode at.
After getting home tonight i happened to watch some pro bike racing and the topic of bad days on the bike was raised in relation to Andre Vinokourov's contrasting performances on two consectives stages of the recently completed Dauphine Libere (sp) and this made me wonder what is the cause of these days where everything just feels wrong when you ride.
I happened to go from having a good day to a bad day, but Vino did the opposite and had a weak followed by a strong day, which in my opinion seems more confusing and harder to explain. At first i thought bad days were caused by tiredness and fatigue and were your body's way of sending a message to pare back a bit, but if it is possible to have a really strong day right after a bad one then this suggests that other factors are involved but i am at a loss to explain what they might be.
Any theories?