Hi: this tree was originally composed entirely of the white and green foliage as it was a variegated cultivar. These sorts of variegations are common in plants and occur spontaneously on lots of different species. They are considered desirable from a horticulture standpoint and so are often put into the nursery trade. But, depending on the plant species and variegation in question, they vary in their long-term stability. In some cases the whole plant remains in the variegated form, or with only a small shoot here and there reverting to green. On others, though you may observe large parts of the plant reverting to the "normal" green form on which the variegation originally occurred. This evidently occurred a long time ago on your tree as much of it is now the green form.