When you control the memory allocation for Minecraft, you really only are configuring the JVM’s garbage collector to use that much memory. That doesn’t include any shared resources outside of the JVM, such as Java itself, OpenGL resources and everywhere else that involves native code and system libraries and drivers.
If you have an integrated GPU, all the textures that normally gets sent to a GPU may also live on your regular RAM too since those use unified memory. That can inflate the amount of memory Java appears to use.
A browser for example, might not have a whole lot of JavaScript memory used, couple MBs maybe. But the tab itself uses a ton more because of the renderer and assets and CSS effects.