snowbird

Hi, Please help identify a lemon tree,it has fruit the size of grapefruit.What is it?

 

Why is it a lemon, and not a grapefruit, or other citrus (there are many kinds)?

 

When the neighbor bought the tree he was told that it was a lemon tree.Its fruit looks like grapefruit. What gives?

 

Could it be a 'Ponderosa' lemon tree? Its fruit has thick, coarse skin and is grapefruit-sized.