The first pic is from the movie, a Sandtrooper, and the other is a pic of a cosplayer. Unless you get a helmet used in the movie it’s going to look different. And even the helmets made for the film didn’t all look identical. However, by the looks of it the cosplayer helmet is an RS propmasters helmet with means it’s a copy of a screen used helmet and as close as you’ll get. Why then, does it look different? Well, depending on how it’s put together, and also in what angle the photo is taken is going to change the look of the helmet. It is also, as you say, depending on were you get your helmet from, if someone is sculpting based on photos or have an actual helmet used in the films to make copies from. As an example, I could make a sculpt and sell helmets and say it’s an ANH helmet but it will not look like the ones in the movie ?
Another ex, from the movie, both are ANH helmets but look very different, the one on the left beeing a Hero helmet and the one on the right is a Stunt helmet.