The M3 version of the Studio can have either an M3 Ultra chip with up to 512GB RAM, or an M4 Max with up to 128. And these aren’t different generations of the product, they were announced at the same time: https://www.apple.com/newsroom/2025/03/apple-unveils-new-mac...
My guess is they’re doing this to solve some yield issue. M4 is a more difficult process with worse yield, so harder to make an Ultra version today. M3 has better yield so it’s tolerable to make an Ultra.
Probably either a tick/tok pattern forming, or a “previous gen gets an ultra version” pattern forming.