I think it's also just a matter of the individual personalities in any particular race, who they're racing against, and how they see their relative strengths and weaknesses. I brought my little boy to the world cup (his first time ever seeing track racing), and he found the first scratch race heat for the woman to be very disappointing. "Why isn't anybody doing anything? Why are they going so slow?" There weren't very many breaks, no one tried to take a lap. I suppose a cynic would say it was a very "ladylike" race.
The second scratch heat was VERY different. Much more aggressive, and a lot more exciting. But then, it was a VERY different group of women.
The scratch race final was MUCH more to my boy's liking. Fast, aggressive, lots of tactics.
In talking about it afterwards, I was explaining to him about the difference between qualifying heats and finals -- you don't half to kill yourself to "win" the heat. You just have to come in the top ten or twelve in order to advance. So, there's not so much competition for that one winning spot. His next question, "How can anyone do two races like that in one day -- a qualifying heat and a final? I do one 5km race (cross-country runner), and I'm dead for the rest of the day!" I told him that's another reason why the qualifying heats are not as aggressive as the finals -- the racers are trying to conserve their energy and strength in order to do the second race.
Also, when you plan your race, you have to take into account your strengths and weaknesses and the strengths and weaknesses of your opponents, and figure out how you can exploit that to your own advantage. Sarah was the overwhelming heavyweight in the race, and I think that skewed everyone's race plan in a completely different direction from what they might have done had she not been there.
The Velodrome Phoenix