Could be as simple as different air pressure in the back tires, or different size, or the former operator always used one corner when digging. If it hasn't worn in the last three years, how much do you think you'll wear before you have to rebuild that part of the bucket? you could add a weld on edge now, or something half assed to keep it going, or leave it, it probably won't be the thing that wears out your machine if you're not using it much and keep an eye on it.