most humans have quite a variation of skin tones across the body. Often the lower arms, face and legs are slightly darker than the torso, as those bits get more sun

also, a great many (ie almost all) poser character skins are made from photos bought from Levius - 3dsk.  So you will find a number of simialr skins, since they started with the same photos.

So its actually quite possible all the skins you notice have dark legs came from photos of one girl who wore shorts a lot that summer. lol

Generally adjusting this in photoshop is not all that difficult, if it bothers you

 

Lyrra