Dwarf ball python?

[asplundii]

What exactly is the difference between the selected stature animals and true dwarfism?

Lirenn pretty much nailed it:

Dwarfism in general is a result of mutations that cause skeletal defects. It's not an exact miniature version of the animal, the organism will grow in certain ways but not others. I would assume the same goes for snakes, but don't quote me on it!

Dwarfism is a single gene mutation that impacts growth, development, and formation of the individual's body. As mentioned, dwarfs as a general rule have physical properties that differentiate them as being not just "miniature versions" of the original in question. Things like "dwarf" burms and retics would be better referred to as "pygmy" or "pygmoid". In stature/stance/build/proportion they match the original but are just scaled down in size.

As a visual example (I find that helps some people) -- Peter Dinklage (4' 6") has dwarfism whereas the people of the Baka tribe are pygmy (average height 4' 8")