(Copypasta from another thread)
If you look at the generic Western and Muslim portraits, you'll notice they are a bit more detailed (and "better") than the portrait pack DLCs, which tend to have smoother skins. However, I think this is more so because the Western and Muslim portraits were made with the original game, so the artists working on those portrait sets had more time to work on them and refine them - as opposed to the portrait DLCs, which I assume have to be made in a shorter amount of time, hence why the artists don't have as much time to work on them (or it's entirely different artists working on them, quite possibly). Some might also notice, for instance, that the portrait DLCs don't differentiate between young and middle aged characters in some cases, and this I think gives more evidence that the artists working on the portrait DLCs have less time and resources at their disposal. Still, I like them though, a little unnerving as they can be.