Kelvin timeline is explained at least by an alteration to the past. The Kelvin was pretty small but the Enterprise, built way later, would have had the most recent tech and whatever. Including stuff that came off of Neros ship. Some of that tech on Neros ship being Borg. There's a whole behind the scenes comic explaining all of it. Super cool.

Discovery… I love the look of the ship. I really do. It's a new and really interesting design and I have been dying for a model for years. Just trying to find a rich sugar daddy who'll get me one. However the explanation for the ship is both extremely interesting to me but also very flimsy for a lot of fans. The explanation is that there is that Utopia Planetia isn't the only shipyard. Now that actually makes sense considering the size of the Federation and the amount of ships in it. The explanation continues that Utopia Planetia is the one that cranks out the most common ships that we see but this alternate shipyard typically works on experimental vessels or prototypes, giving them way more leeway with design. It also being done by an entirely different crew so they didn't have the same design ideals that Mars did. I think that's a fucking awesome explanation that's really interesting and makes sense (even for prototypes like Prometheus in Voyager) but a lot of people did not like that at all.