Why are there different chemical structures for epinephrine (adrenaline)?
There are often shortcuts taken in drawing complicated molecules, particularly if there's not any interesting chemistry going on at those functional groups.

The $\ce{-CH3}$ is implied at the end of the "stick," just like how the carbon atoms aren't explicitly labeled in the benzene ring.

Hydrogens are rarely explicitly drawn for the same reasons, given that you can infer the hybridization by inspection.

See Skeletal formula for more details.  Briefly:


  Carbon atoms are usually depicted as line ends or vertices with the assumption that all carbons have a valence of 4 and carbon-hydrogen bonds, usually not shown explicitly, are assumed to complete each C valence. A skeletal formula shows the skeletal structure or skeleton of a molecule, which is composed of the skeletal atoms that make up the molecule.