Shane, since you're teaching English in China, have you noticed a difference that:
In China, the lunch is the richest one, at which people have not only more food but more nutritious food. And they believe it is a healthy life style to eat less in the evening.
In North America (I guess in UK it is the same) people don't eat much at lunch time, but rely on the meal in the evening for a full stomach.
My point is, Chinese think
1. dinner = chief meal in the day
2. evening meal <> chief meal
So they would rather use 'supper' than 'dinner'.