Answer
The later transplant happened past a point of differentiation, where the transplanted cells had already been triggered to develop into skin.
Work Step by Step
You could test this hypothesis by doing repeat transplants from different age salamanders to the same age tadpoles to find if a clear line of demarcation exists. If the above hypothesis is correct, you would discover a certain age or point in development at which the transplanted cells would always turn into salamander skin.
