Sizes 6, 6X, and 7 are intermediate sizes. They are for the girl who is not a toddler anymore but not large enough for the young ladies sizes of a medium. Often times a 4-5 is a x-small, the 6-7 a small, and then the 8-10 is medium, and 12-14 large, 16 x-large. The truth is that a girls size large is often the same size as a ladies x-small and the x-large the same as a ladies medium.
The girls sizes have smaller leg openings, smaller arm holes, more straight up and down in the hip area and less curves. The ladies will often have a fuller leg in pants and jeans, swim suits will have a high cut thigh and less coverage in a bikini top, The arm holes will be larger, the hip area will be curvier to accommodate women who have had children and their hips have widened, and the ladies shirts will have fuller breast areas with darts.
They do this the same as with the odd sizes of girls shoes. A girls size 4 is the same as a ladies size 7-8. They make them look girlish and not hoochie-mama so that the girls will still like them but make them for girls to wear as their bodies get larger but they are still little girls.
So, when buying clothes always have them try on things. Each and every designer has their own ideas about what a size is, they also have Taiwan and China and Mexico and any other country that will give them a great deal make them.
When you look at a garment and compare even the same size and style you'll get much different lengths and widths. In the store that I work in we have leotards from name brand companies. I can go over and pull the same size and style of leo off the rack and one will be 4 inches longer than the other one I am holding. They are from the same company and just different shipments. They came from different countries.
Always try on clothes and don't assume a size and style of anything is going to be the same from pair to pair.