Lemon or Satsuma?

This Satsuma tree has been a special one to watch over the past few years.  It was a gift from dear friends when we first built the house in Georgia.  Last year we had our first crop - 5 yummy satsumas.  Imagine our surprise this year when we discovered the tree laden with fruit.  Most of it looked like lemons, but there were two definite satsumas on the bottom branch.

I can absolutely guarantee this fruit is satsuma - no seeds, juicy and delicious.

From the higher branches of the same tree - this obvious lemon.  My friend made lemonade with some of these, and said it was delicious.

Here they are all together - honestly, they came off the same tree!
As a clue - we used to have two satsumas - the older one died last summer in the drought.  Could it be a pollination thing?