While trying to generate a feature of the "fossil" type with a given height, I came across the fact that, regardless of the used modifiers associated with the generation height, the structures are distributed in their own way. By setting "height_range" as a constant, the structure is generated at a different height than the specified one, and with varying heights. I checked this with structure, which is empty space with one block height.

Location of the object increases the height after increasing height in file. For type "uniform": if max y value and min y value are equal, then result will be same; if difference is non-zero, then the scatter will increase. Also, the structure is always generated higher than 9 blocks above bottom.

Example (see screenshots and data pack): I set modifier for placed feature of the "fossil" type

"height":{"type":"minecraft:constant","value":{"absolute":60}}

Expectation: objects locate only on y=60.

Reality: objects locate on y from 36 to 45.