It's not odd, people just don't think about the 'why' and jump to conclusions around sponsorships instead. There's really three reasons why:
1. The game is still based around their same engine since Black Flag (and it + Unity had heavy Nvidia involvement during dev in fact) which is suited better to graphics-heavy archs, like Nvidia used to have and AMD didn't. Kepler & Maxwell being obvious examples compared to the forward-looking & compute-focused GCN.
2. Today that's flipped on its head: Nvidia is more heavily compute-oriented while RDNA 2 is better at graphics. Further, Nvidia's advantages with Tensor & RT cores are not relevant here because there's no DLSS, and no RT.
3. AC games suffered immensely on PC due to DX11 (and various poor programming practices on their part, cf Kaldaien), and AMD also did poorly in DX11 titles due to various reasons but here Nvidia's software team had a decisive advantage, furthermore AC games became more & more demanding on CPU & memory. Valhalla is now DX12-only, so in a way AMD cards are now unshackled and can run properly and thus we see a big advantage for them here.
You can see a similar thing with Wildlands vs Breakpoint (they also use same engine as AC), when they finally added Vulkan to BP it was a huge boon for AMD users. Wildlands in particular was horrific and you could see this the best if you paid any attention to the power usage. It was always low on AMD cards, which is indicative of underutilisation. I remember with my Vega 64 I'd see the SOC power get cut in half almost, and that's irrespective of settings/reso. It was... wild. Mind you, it wasn't perfect for NV gpus either but it was much better than for AMD. The difference between RDNA 2 & Ampere isn't as big for BP, but it's there, and that's also at heart very much a DX11 title still, and the game also had more use for compute.