Naranja o naranjo

Naranja o naranjo
 

I've always seen naranja and manzana but just hit naranjo and manzano in Rosetta Stone. They are showing apple and orange trees full of fruit.

Just kind of seems strange.

One is the fruit. One is the tree. La naranja es el fruto del naranjo.

There are other pairs of fruit and fruit tree names that fit the pattern ~a for the fruit and ~o for the tree (la toronja, el toronjo [grapefruit, grapefruit tree]). There are also many pairs of names that don't fit this pattern, such as la pera, el peral (pear, pear tree) and el higo, la higuera (fig, fig tree).

Thank you. I suspected the fruit/tree thing but since Rosetta Stone has NO explanations I wasn't sure. I'm enjoying the total immersion concept of Rosetta Stone but it's hard for me to just accept and not question why.