Isn't it the same person? It says the author of the WN is Resn and Resn is listed as the author of the manga too. (and seems the artist, Yachimoto, is the same for both too?)
If so then they are seemingly just adding things they couldnt fit, or didnt think of at the time, into their story which should be fine. Usually LNs or manga are the finalized version of a WN. Kind of similar situation with the differences between ReZero's WN -> LN.
resn is the original author, but they don't do any of the writing in the manga - that's all yachimoto. That being said, resn has been pretty vocal on twitter about how they like all of the new additions. Personally, I think it's doing a lot to give depth to characters that were otherwise kinda just plopped in without much thought in the WN or never really had their own arcs.
Ultimately, it's the same story, just being told in different ways. The WN is a lot faster, and some people want that (and feel the need to complain here for some reason). A good comparison would be that the DUO WN is like the first arc of SAO, and the DUO manga is like SAO progressive. Somewhat rushed in the source material, so it's getting fleshed out and really solidifying the base of the story.
For what it's worth, yachimoto has mentioned on twitter that the pace will get faster once the current overarching tournament arc finishes.