The jumpsuit uniforms created for the first season had zippers on the front. When the newer uniforms (which zipped up in the back) were introduced in season 3, the older style uniforms continued to be used by background characters. Sometime after that, these uniforms were modified to more closely resemble the newer uniforms, however, they retained their front zipper. Interestingly this is also why some of the Science/Medical uniforms are a different color than others, as the new uniforms were made using a different fabric than the old ones.
On at least one occasion, a combination of the two styles (and some fancy camera work) was used to allow a crew member to open a uniform that appeared to be of the newer style from the front, as if by a hidden 24th century closure