What I meant in specific is that the trailers are typically just in-game recorded footage. They construct the scenes in game, spawning mobs to do animations in game, because they're pretty much always moving camera shots showing off bits of the zones from different angles and stuff (and overlaying moving things on moving backdrops with moving cameras is a recipe for fake looking).
Here you have four nearly identical shots with mobs basically machinima'd on. By the look of the blur around the models, flat PNGs that were pasted over top of the picture of the landscape and then had to be blurred because they weren't alpha key'd into a scene, or cropped out of their model viewer image well.
Blizzard does some of that compositing in those still press shots, but this is pretending to be a trailer so it shouldn't have them, and if it did have them they should be
higher quality without blurring, because Blizzard's trailer team has the software to just greenscreen the models directly on rather than photoshopping them into place.