Yes, it was the seed that remained viable through the winter. It is interesting how in nature things do grow very rapidly. When the soil reaches the proper temperature, they take off. But, the hothouse plants that have tomatoes on them when you are just planting seed is going to produce a crop before those naturally planted.
With trees that are potted and left for a couple of years, they do not gain much, if any over a seedling plated at the same time in the ground. That is, if you plant a seedling in the ground and a 15 gallon tree that is three years old, you are not going to see much difference over time. The seedling placed in the ground will grow more rapidly than the potted tree. In the long run, I am told that the seedling will probably be the better tree. Lesson....plant your seedlings now instead of buying a large expensive potted fruit tree.