Race results are not only determined by how fit you are. Were the courses comparable? Was the weather different? Were you comparably rested when you went to sleep on the night before the race, etc.? Any of those kinds of things could affect your performance and if some of these sorts of things were more "negative" for the second race than the first that may mean that you actually improved more in the second race than the time suggests.
Your training details here are fairly sketchy. Did you distribute your mileage the same way for each race, e.g, did you get that volume in mostly in the same number of runs each week or did you, say, spread it over seven runs a week for one race and five, six, ten, whatever, for the other? Did you run your intervals and tempos faster for the either race? Did you race at other distances in your lead up to each race and if so did you see progress in your performances in the ones that came before the second race compared to the ones from before the first one?
Finally, how long before each race did you average those 90 km weeks for? If you had a couple of rest weeks after the first race and then got right back to your 90 km weeks you'd likely get a different result than if you dropped way down for eight or nine months and then got back to the 90s until race day. A year of consistent training should get you significantly fitter and faster even if you're just doing the same thing but that's not guaranteed.
If you did the second thing, you probably lost a good bit of fitness and it wouldn't be unexpected that you'd pretty much just get back to where you were. If you did the first, it may be that you got about as fit as you are going to on the amount of training that you're doing. assuming all those sorts of factors I mentioned in the first paragraph were similar for each race. I found that to make significant improvements in my performances I needed to make significant increases in how much I trained and that MAY be the thing you'll need to do in order to get off of your plateau.