From what I heard from Lego, they were planning on switching all their slopes to being smooth, but then changed their minds a couple years later, deciding that all slopes would have some texture.
I'm not sure why, but my guess would be that for sticker application, smooth works MUCH better-- and they're doing a lot more stickers now than they did in years past as a cost-saving measure. It also did look a little strange that some slopes were smooth (2x2x2, 6x1x5, and 6x8x1), while the rest were textured. But for roofing, I expect that the textured roofs look far better.
Puddleglum wrote:As for your question about whether they are "officially different parts", I'm not sure if there's really a good answer to that question.
Yeah, I think it depends on what department you talk to at Lego. Technically, there are a bunch of different reference numbers for a particular element. So the "design ID" is the same across the board, but some of the other identifiers might not be (I think "part number" is probably still the same). Certainly in terms of packing, they were considered by Lego to be identical, since you could receive both smooth and textured slopes in the same color for the same elements for the SAME SET. But if you talked to the manufacturing departments, I'm sure they'd easily make a distinction between them. In fact, they make a distinction between each individual chamber of a particular mold!
As for the texture itself, it does indeed come in a lot of different varieties. The oldest texturing that I've seen looks somewhat like itty bitty craters scattered in with the texture, and is indeed rougher than current textures. I'm not sure how many grades of textures there have been, but it's probably been quite a few. And certainly they seem to have universally gotten smoother and smoother as the years have gone on (apart from the migration from actually smooth surfacing that was done recently). Some have suggested that this may be due to wear on the molds, but that seems very unlikely to me (although I don't think Lego has ever commented on the possibility).