Byron Tie great question - through our literature review we found evidence (this is in Australia mind you) that about 64% of food waste (by weight) is avoidable, with most of that being bakery goods, vegetables and prepared meals!
About 11% is possibly avoidable, and 24% was unavoidable.
Thanks Kate - very similar to @Ben Odongo. For example, young people that live alone tend to spontaneously change their plans (so meal planning for the week isn't always appropriate).
There was a story of a young mother who always over purchased at the supermarket as she had very picky children so she needed to have multiple options to offer them each night.
There were instances of people forgetting about the leftovers that they had in the fridge and once a certain amount of time had passed they were deemed 'inedible' and disposed of (even though they might have been ok).
Another one was that a household would get sick of eating the same leftovers after a day and would (for example) go out for lunch instead. So overcooking was a problem there.