In the tiling layout, niri is constantly asking windows to assume their expected size. In contrast, floating windows should be able to freely change size as they see fit.
The logic turns out to be quite tricky. On the one hand we want a window to keep its latest size, but on the other we still want to be able to resize the window, which means asking it for a different size. The window can take a second to respond, or respond with a yet another size, and nothing must break.