Tree looks great. I agree that the fruit might be from the rootstock. Generally there should be some fruit on the secondary branches close to the main trunk. I could not find anything higher up. Are there fruit in the canopy? I remember someone posting that one of their Excalibur trees was grafted around 2 ft up. I would also thin that low cluster down to 3 - 4 fruits, one or two per stem connected to the trunk. I cant tell the size of the tree from the picture but heavy fruiting can be hard on a 3 year old tree. The young tree can handle no more than one or two fruit on each major secondary limb.