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Yeah I think this is a big part of it. I felt tired, and despite putting out near identical power (AP/NP) to my previous go on that course, I was 15w down on the SB power that I set on Wednesday. I think that the tiredness, and the associated low motivation meant that I was lazy and allowed my power slump on the difficult sections of the course while I still managed to pushed on a bit during the easy sections.
Slacking off in the hard times is exactly the opposite of what I should have been doing.
Average power doesn't tell you where you delivered the power. You might've pushed harder into the wind or hills last time and slacked off more elsewhere.