The difference is Sienkiewicz does indeed make a stylistic choice. Liefeld is simply uneducated and unskilled. He's an amateur, and one without any evident talent, at that. His work is that of the average pre-teen.
See my avatar? I did it, in the fashion of Greg Land. It's part of a larger work in progress. It's hack work, but at least it's competent. Liefeld is incompetent.