There are multiple issues in your question
- You are using the "source colour" (#000000) but not a "primary color"
to generate a theme
- You cannot ensure that the generated primary color exactly matches your specified color and should not be expecting this
Answering your first question:
- After a number of calculations library creates a TonalPalette
based on the source color
- Then it picks from the palette tone(50) color for light
theme as a primary color
- For more details you can explore source code https://github.com/material-foundation/material-color-utilities/blob/main/typescript/utils/theme_utils.ts#L77