Interesting that you say 99%.. I know it's a figure of speech but in my experience people are very unaware of how much of a positive effect they can have on their own lives, just by changing the way they behave and think about certain things. For example, it have been shown that sitting or standing in "powerful" poses for as little as 2 minutes can have serious effects on the testosterone and cortisol levels produced manifesting in a far more confident, less stressful, demeanour*
Simply focusing on something can have serious implications, most people that want to become more healthy will say things like "I'm not going eat chocolate" and invariably spend a lot of time thinking about the fact they are "not going to eat CHOCOLATE" i.e. they spend a lot of time thinking about chocolate. The people that say "I'm going to eat more apples" or "I'm going to eat healthy food" will generally have a greater degree of success because they are not focusing on the negative they wish to amend (There is a whole field of positive psychology, Martin Seligman being a pioneer in this field if you want to know more about the work done)
I would say people are capable of a lot more change than they realise, it's all about changing habits, the more aware people are of how they should go about changing these habits, the more success they are likely to have. Will power is too vague (as far as I am aware) to be measured but it obviously varies in people depending on their experiences, but everyone has the ability to change who they are and the number and depth of the habits they wish to amend and how much they know about changing them will play as much of a part in this.
By depth I mean how ingrained the behaviours are and this will be based on the groups of people they associate with, the length of time this habit has been a habit, but the biggest problem for most people is identifying the 'bad' habits. Obviously in terms of eating and smoking it's obvious (bad food and cigarettes), but habits involving general ambition to be happier or more fulfilled are a little harder to pin down. Anyway, that's my explanation for my most people that are miserable stay so and most people that eat bad food and smoke tend to continue do to so. Not so much will power as knowledge and social influences. Will power might even be a word we have developed to explain the complex art of establishing the most effective means of achieving positive change in life, more than a "measurable thing" in its own right.