Yeah, looking it up, the Switch version had enough improvements and changes I easily see why Deluxe got scored higher than the Wii U release:
- All DLC included upfront
- Fully redesigned battle mode, including all new modes and maps
- New characters
- Item rebalancing (apparently you could only hold one item at a time on Wii U!)
- Performance fixes (Wii U version skipped the 60th frame every second; Wii U version ran split screen at reduced frame rates)
- 200cc mode
Then of course, with updates, even more got added to it – Sound Tester, more characters, more kart parts, double the number of tracks, Amiibo and Labo support, etc. Makes total sense that a game that was already that high scoring to begin with getting a very improved release relatively quickly saw the improved release get a higher score. We saw this happen with Persona 5, Street Fighter 4, and Ghost of Tsushima recently as well, so if any "suspicious conspiracy" exists like vaibhav is suggesting, it seems to apply to pretty much all games and publishers, not just Nintendo and Mario Kart.