I'm not sure if you reduced the quality of the original picture to upload it, the model is running a low rez texture, or you have that setting turned on in sfm.
For a while, I haven't been using sfm, but a while ago. I went back on and noticed all my projects were pixelated!!!!!! So I was hella pissed and I recall you having to right click and some setting. I'll try to check it again. But it's basically some strange kind of lighting and if used right you can make the graphics like the walking dead where every model is outlined to the camera's perspective with black.
It'd be better if you uploaded the model to see whether there's something wrong with your sfm or your models. Also, if your models aren't created from scratch they may have some funny settings. Like the vertices aren't connected, etc. But half and half is quite rare.
I'm not sure if you're saying it's always like that or if you rotate the model, the half dark/brighter lighting is elsewhere. If it's in the same spot, then that's different.