We've used a combination of Next Level 6010, 6210 and 6410 for years; each of the lines has some subtle differences in color and finish, so that's usually the primary focus. The only difference with the DTG shirts is that they're printed on demand (and the tags are left in to identify the size), versus ordered in bulk from our screen printer, who replaces the tags with a print.
That said, it's not uncommon to have some variances in fitment - I've got three of the Blood Red shirts, and they all fit slightly different. Sorry the trio of shirts you got weren't up to you expectations.
