to be honest if i look at your models and these, i am not seeing a big difference in quality. i see difference in style. some of those examples have more details put into the models, but they go a little more towards realism. some of them have more contrast in the lighting. some of them have a little higher production…
the trouble usually come from the miss matched normals (when you stitch in the geometry this is not an issue) at the seam. for example.... the cyinder on the right has the normals at the seam edited to be the same as the plane.